Improve code coverage of the main library

This issue calls for improving code coverage of the main library.

A code coverage report was created by executing `cabal test --enable-coverage lsm-tree:lsm-tree-test` on the `jeltsch/unsliced-keys-in-ordinary-indexes` branch as of 28 March 2025, that is, on commit dd6b71c95ff426684792a63b91cf15435bdaf61d.

The individual coverage deficits of all modules in the main library where examined. All non-trivial cases of missing coverage are listed below. Each case is described by either of the following:

* A situation that never occurred, which led to code not being executed
* A value that was never used (in which case the description contains the phrase “never used”)

Coverage deficits of the following kinds were considered trivial and are therefore not mentioned here:

* Derived class instantiations
* Instantiations of `NFData`
* Field selectors
* Error message strings or parts thereof
* Deliberately unused values[^deliberately-unused] (where it was clear that they are deliberately unused)
* Explicit `()`

[^deliberately-unused]:
An example is the `index` argument in the second alternative of `Database.LSMTree.Internal.Index.finalLBS`, which is not used by the underlying function `Database.LSMTree.Internal.Index.Ordinary.finalLBS`.

`Data.Arena`
------------

* `scrambleArena`
- [ ] Non-empty free list
* `newBlockWithFree`
- [ ] Non-empty free list

`Data.Map.Range`
----------------

* `rangeLookup`
- [ ] Unbounded in at least one direction
* `evalLowerBound`
- [ ] Exclusive lower bound

`Database.LSMTree`
------------------

* `Bifunctor LookupResult`
- [ ] Never used
* `Bifunctor (QueryResult k)`
- [ ] Never used
* `rangeLookup`
- [ ] Resolver never used
* `withCursor`
- [ ] Never used
* `withCursorAtOffset`
- [ ] Never used
* `readCursor`
- [ ] Resolver never used
* `retrieveBlobs`
- [ ] Invalid blob reference
* `unions`
- [ ] Table type mismatch

`Database.LSMTree.Common`
-------------------------

*Nothing*

`Database.LSMTree.Internal`
---------------------------

* [ ] All trace messages never used
* `withOpenSession`
- [ ] Session is closed
* `openSession`
- [ ] Unexected error when acquiring session file lock
- [ ] Session directory locked with `tryLockFile` returning `Nothing`
- [ ] Registry never used for restoring session
- [ ] Active directory never used when removing it in session creation
- [ ] Snapshots directory never used when removing it in session creation
- [ ] Corrupted snapshots directory when restoring session
- [ ] Non-empty active directory when restoring session
- [ ] Session already closed when closing session
- [ ] Rollback for cursor list acquisition when closing session
- [ ] Non-empty list of cursors when closing session
- [ ] Rollback for table list acquisition when closing session
* `newEmptyTableContent`
- [ ] Registry never used for creating levels cache
* `rangeLookup`
- [ ] Resolver never used
- [ ] Multiple chunks
* `readCursor`
- [ ] Resolver never used
* `readCursorWhile`
- [ ] Resolver never used
* `createSnapshot`
- [ ] Named snapshot directory never used when rolling back snapshot directory creation
* `openSnapshot`
- [ ] Wrong table type
- [ ] Resolver never used when opening write buffer
* `wrapFileCorruptedErrorAsSnapshotCorruptedError`
- [ ] Ordinary action registry abort (`ExitCaseAbort`)
- [ ] Commit action registry error
- [ ] Unmentioned exception
* `doesSnapshotExist`
- [ ] Never used
* `listSnapshots`
- [ ] Alleged snapshot directory not being a directory
* `unions`
- [ ] Table session mismatch
- [ ] Session closed
- [ ] Unmodified session state after creation of unions table never used
* `writeBufferToNewRun`
- [ ] Level number 1 never used
* `matchSessions`
- [ ] Session root mismatch
* `supplyUnionCredits`
- [ ] Union level denotation never used

`Database.LSMTree.Internal.Assertions`
--------------------------------------

* `fromIntegralChecked`
- [ ] Conversion error found

`Database.LSMTree.Internal.BitMath`
-----------------------------------

* `mod2`
- [ ] Never used
* `ceilDiv2`
- [ ] Never used
* `mod4`
- [ ] Never used
* `div16`
- [ ] Never used
* `mod16`
- [ ] Never used
* `mul16`
- [ ] Never used
* `divPageSize`
- [ ] Never used
* `modPageSize`
- [ ] Never used

`Database.LSMTree.Internal.BlobFile`
------------------------------------

* `openBlobFile`
- [ ] Error when opening the file

`Database.LSMTree.Internal.BlobRef`
-----------------------------------

* `deRefWeakBlobRef`
- [ ] Invalid reference

`Database.LSMTree.Internal.BloomFilter`
---------------------------------------

* `bloomFilterFromSBS`
- [ ] Wrong endianness
- [ ] Bloom filter too large
- [ ] Byte array too large for components

`Database.LSMTree.Internal.BloomFilterQuery1`
---------------------------------------------

* `Show RunIxKeyIx`
- [ ] Never used

`Database.LSMTree.Internal.BloomFilterQuery2`
---------------------------------------------

* [ ] All assertion properties never used
* `alignmentOfType# @CandidateProbe`
- [ ] Never used
* `indexOffAddr# @CandidateProbe`
- [ ] Never used
* `Show CandidateProbe`
- [ ] Never used

`Database.LSMTree.Internal.ByteString`
--------------------------------------

* `tryCheapToShort`
- [ ] Never used
* `tryGetByteArray`
- [ ] Non-zero offset into byte array
- [ ] Contents referenced by a `MallocPtr`
- [ ] Contents referenced by a `PlainForeignPtr`
- [ ] Contents referenced by a `FinalPtr`
* `unsafePinnedByteArrayToByteString`
- [ ] Byte array not pinned
- [ ] Contents pointer never used

`Database.LSMTree.Internal.CRC32C`
----------------------------------

* `hGetExactlyCRC32C_SBS`
- [ ] Contents pointer never used
* `hGetAllCRC32C'`
- [ ] Non-positive chunk size
- [ ] Contents pointer of internally used byte string never used

`Database.LSMTree.Internal.ChecksumHandle`
------------------------------------------

*Nothing*

`Database.LSMTree.Internal.Chunk`
---------------------------------

*Nothing*

`Database.LSMTree.Internal.Config`
----------------------------------

* `runParamsForLevel`
- [ ] Level number never used
* `Semigroup TableConfigOverride`
- [ ] Never used
* `Monoid TableConfigOverride`
- [ ] Never used
* `configOverrideDiskCachePolicy`
- [ ] Never used
* `bloomFilterAllocForLevel`
- [ ] FPR request
* `diskCachePolicyForLevel`
- [ ] Caching from a given level downwards

`Database.LSMTree.Internal.Cursor`
----------------------------------

* `readEntriesWhile`
- [ ] Mupsert with a key that has other entries

`Database.LSMTree.Internal.Entry`
---------------------------------

* `hasBlob`
- [ ] Never used
* `onBlobRef`
- [ ] Never used
* `bifoldMap @Entry`
- [ ] Blob reference not used when inserting with blob
* `combineMaybe`
- [ ] Never used

`Database.LSMTree.Internal.IncomingRun`
---------------------------------------

* `nominalDebtAsCredits`
- [ ] Never used
* `supplyCreditsIncomingRun`
- [ ] Level number never used when computing threshold
* `depositNominalCredits`
- [ ] Previous nominal credits never used
* `immediatelyCompleteIncomingRun`
- [ ] Level number never used when supplying credits

`Database.LSMTree.Internal.Index`
---------------------------------

*Nothing*

`Database.LSMTree.Internal.Index.Compact`
-----------------------------------------

* `search`
- [ ] Empty index
- [ ] Sequence of large-entry pages and pages with clashes without preceding pages found
- [ ] Search key belongs to the second or later page in a series of clashing pages[^clash-absence-hint]
* `hasClashes`
- [ ] Never used
* `toLBS`
- [ ] Never used
* `putBitVec`
- [ ] Bit vector not byte-aligned
* `fromSBS`
- [ ] Wrong endianness
- [ ] Number of pages not representable as `Int` value
- [ ] Number of entries not representable as `Int` value
- [ ] Byte array too large for components
* `checkedPrimVec`
- [ ] Negative offset
* `checkedBitVec`
- [ ] Negative offset
* `bitIndexFromToRev`
- [ ] No upper bound
* `vectorLowerBound`
- [ ] No lower bound
* `vectorUpperBound`
- [ ] Exclusive upper bound

[^clash-absence-hint]:
The existence of this case in connection with the previous one suggests that there might be no clashes at all.

`Database.LSMTree.Internal.Index.CompactAcc`
--------------------------------------------

*Nothing*

`Database.LSMTree.Internal.Index.Ordinary`
------------------------------------------

* `fromSBS`
- [ ] Wrong endianness
- [ ] Number of entries not representable as `Int` value

`Database.LSMTree.Internal.Index.OrdinaryAcc`
---------------------------------------------

*Nothing*

`Database.LSMTree.Internal.Lookup`
----------------------------------

* `intraPageLookupsOn`
- [ ] Combining of entries never used (two occurrences)
- [ ] Byte count discrepancy

`Database.LSMTree.Internal.Merge`
---------------------------------

* `abort`
- [ ] Implicitly closed merge
- [ ] Explicitly closed merge
* `complete`
- [ ] Ongoing merge
- [ ] Implicitly closed merge
- [ ] Explicitly closed merge
* `stepsToCompletion`
- [ ] Never used
* `steps`
- [ ] Completed merge
- [ ] Implicitly closed merge
- [ ] Explicitly closed merge
* `doStepsLevel`
- [ ] Readers drained when handling mupdate
- [ ] Readers drained when dropping remaining entries of the same key

`Database.LSMTree.Internal.MergeSchedule`
-----------------------------------------

* [ ] All trace messages never used
* `mkLevelsCache`
- [ ] Reference release operation never used in rollback for duplicating runs
* `iforLevelM_`
- [ ] Level number never used by consumer
* `flushWriteBuffer`
- [ ] Empty table write buffer
- [ ] Level number 1 never used
* `addRunToLevels`
- [ ] Level number never used when immediately completing incoming run
* `mergingRunParamsForLevel`
- [ ] Regular level specification never used
* `maxRunSize`
- [ ] Negative level number
* `mergeTypeForLevel`
- [ ] Last-level denotation never used
* `supplyCredits`
- [ ] Level numbers never used

`Database.LSMTree.Internal.MergingRun`
--------------------------------------

* `new`
- [ ] Empty new merge
* `newCompleted`
- [ ] Error handler never used
* `unsafeNew`
- [ ] Table too large
* `atomicModifyInt`
- [ ] CAS looping
* `supplyCreditsAbsolute`
- [ ] Information on supplied credits never used (two values)
* `performMergeSteps`
- [ ] Merge completed
* `completeMerge`
- [ ] Merge completed
* `expectCompleted`
- [ ] Merge maybe completed
- [ ] Merge ongoing

`Database.LSMTree.Internal.MergingTree`
---------------------------------------

* `expectCompleted`
- [ ] Ongoing tree merge
- [ ] Pending tree merge

`Database.LSMTree.Internal.MergingTree.Lookup`
----------------------------------------------

* `mkLookupNode`
- [ ] One lookup tree
* `mergeLookupAcc`
- [ ] Resolver never used when merging levels
* `buildLookupTree`
- [ ] Reference release operation never used in rollback for duplicating runs (two occurrences)
- [ ] Pending level merge with no merging runs but a union merge

`Database.LSMTree.Internal.Page`
--------------------------------

*Nothing*

`Database.LSMTree.Internal.PageAcc`
-----------------------------------

*Nothing*

`Database.LSMTree.Internal.PageAcc1`
------------------------------------

* `singletonPage`
- [ ] Deletion

`Database.LSMTree.Internal.Paths`
---------------------------------

* `isValidSnapshotName`
- [ ] String contains a character that is neither a lowercase Latin letter nor a digit
* `toSnapshotName`
- [ ] String is not a valid snapshot name
* `pure @ForRunFiles`
- [ ] Never used

`Database.LSMTree.Internal.Primitive`
-------------------------------------

*Nothing*

`Database.LSMTree.Internal.Range`
---------------------------------

*Nothing*

`Database.LSMTree.Internal.RawBytes`
------------------------------------

* `IsList RawBytes`
- [ ] Never used
* `unpack`
- [ ] Never used
* `unsafeFromByteString`
- [ ] Byte array fetching unsuccessful

`Database.LSMTree.Internal.RawOverflowPage`
-------------------------------------------

*Nothing*

`Database.LSMTree.Internal.RawPage`
-----------------------------------

* `getRawPageIndexKey`
- [ ] No index

`Database.LSMTree.Internal.Run`
-------------------------------

* `Show (Run m h)`
- [ ] Never used
* `runFsPathsNumber`
- [ ] Never used
* `mkRawBlobRef`
- [ ] Never used
* `openFromDisk`
- [ ] Invalid file (three occurrences)
- [ ] Unexpected checksum

`Database.LSMTree.Internal.RunAcc`
----------------------------------

* `addKeyOp`
- [ ] Chunks emmitted by small key/op addition with page emission never used

`Database.LSMTree.Internal.RunBuilder`
--------------------------------------

*Nothing*

`Database.LSMTree.Internal.RunNumber`
--------------------------------------

*Nothing*

`Database.LSMTree.Internal.RunReader`
-------------------------------------

*Nothing*

`Database.LSMTree.Internal.RunReaders`
--------------------------------------

* `Eq (ReadCtx m h)`
- [ ] Never used
* `nextReadCtx`
- [ ] Reinstantiated empty key/op buffer never used

`Database.LSMTree.Internal.Serialise`
-------------------------------------

* `sizeofKey64`
- [ ] Never used
* `sizeofValue16`
- [ ] Never used
* `sizeofValue64`
- [ ] Never used
* `serialisedValue`
- [ ] Never used
* `SerialisedBlob`
- [ ] Never used (at least not in expressions)
* `serialisedBlob`
- [ ] Never used

`Database.LSMTree.Internal.Serialise.Class`
-------------------------------------------

* `serialiseKeyMinimalSize`
- [ ] Never used
* `requireBytesExactly`
- [ ] Unexpected number of bytes
* `deserialiseKey @Word64`
- [ ] Unexpected number of bytes
* `deserialiseValue @Word64`
- [ ] Unexpected number of bytes
* `serialiseValue @Void`
- [ ] Never used
* `deserialiseValue @Void`
- [ ] Never used

`Database.LSMTree.Internal.Snapshot`
------------------------------------

* `fromSnapMergingTree`
- [ ] Completed tree merge
- [ ] Pending level merge
* `snapshotWriteBuffer`
- [ ] Rollback for write buffer writing
* `openWriteBuffer`
- [ ] Resolver never used
* `fromSnapLevels`
- [ ] Level number never used
* `fromSnapMergingRun`
- [ ] Error handler never used

`Database.LSMTree.Internal.Snapshot.Codec`
------------------------------------------

* `prettySnapshotVersion`
- [ ] Never used
* `readFileSnapshotMetaData`
- [ ] Invalid file
* `decodeSnapshotMetaData`
- [ ] Exception
* `decode @(Versioned a)`
- [ ] Incompatible snapshot format version
* `decode @SnapshotVersion`
- [ ] Unknown snapshot format version
* `decodeVersioned @SnapshotTableType`
- [ ] Unexpected tag
* `decodeVersioned @SnapshotRun`
- [ ] Unexpected combination of list length and tag
* `decodeVersioned @MergePolicy`
- [ ] Unexpected tag
* `decodeVersioned @SizeRation`
- [ ] Not 4
* `decodeVersioned @WriteBufferAlloc`
- [ ] Unexpected tag
* `decodeVersioned @RunParams`
- [ ] Unexpected combination of list length and tag
* `decodedVersion @RunDataCaching`
- [ ] Unexpected tag
* `decodedVersion @IndexType`
- [ ] Unexpected tag
* `decodedVersion @RunBloomFilterAlloc`
- [ ] Unexpected combination of list length and tag
* `decodedVersion @BloomFilterAlloc`
- [ ] Unexpected combination of list length and tag
* `decodedVersion @FencePointerIndex`
- [ ] Unexpected tag
* `decodedVersion @DiskCachePolicy`
- [ ] Unexpected combination of list length and tag
* `decodedVersion @MergeSchedule`
- [ ] Unexpected tag
* `decodedVersion @SnapIncomingRun`
- [ ] Unexpected combination of list length and tag
* `decodedVersion @MergePolicyForLevel`
- [ ] Unexpected tag
* `decodedVersion @SnapMergingRun`
- [ ] Unexpected combination of list length and tag
* `decodedVersion @LevelMergeType`
- [ ] Unexpected tag
* `decodedVersion @TreeMergeType`
- [ ] Unexpected tag
* `decodedVersion @SnapMergingTreeState`
- [ ] Unexpected combination of list length and tag
* `decodedVersion @SnapPendingMerge`
- [ ] Unexpected combination of list length and tag
* `decodedVersion @SnapPreExistingRun`
- [ ] Unexpected combination of list length and tag

`Database.LSMTree.Internal.StrictArray`
---------------------------------------

*Nothing*

`Database.LSMTree.Internal.UniqCounter`
---------------------------------------

*Nothing*

`Database.LSMTree.Internal.Unsliced`
------------------------------------

* `unsafeMakeUnsliced`
- [ ] Never used
* `unsafeMakeUnslicedKey`
- [ ] Never used
* `Show (Unsliced SerialisedKey)`
- [ ] Never used

`Database.LSMTree.Internal.Vector`
----------------------------------

* `mapMStrict`
- [ ] Never used
* `imapMStrict`
- [ ] Never used
* `zipWithStrict`
- [ ] Never used

`Database.LSMTree.Internal.Vector.Growing`
------------------------------------------

* `new`
- [ ] Non-positive buffer size
* `append`
- [ ] Length overflow
- [ ] Only one possible new buffer size
- [ ] No possible new buffer size

`Database.LSMTree.Internal.WriteBuffer`
---------------------------------------

* `fromList`
- [ ] Never used
* `lookups`
- [ ] Never used
* `rangeLookups`
- [ ] Never used
* `convertRange`
- [ ] Never used

`Database.LSMTree.Internal.WriteBufferBlobs`
--------------------------------------------

*Nothing*

`Database.LSMTree.Internal.WriteBufferReader`
---------------------------------------------

* `readWriteBuffer`
- [ ] Resolver never used
* `new`
- [ ] Error handler never used (two occurrences)

`Database.LSMTree.Internal.WriteBufferWriter`
---------------------------------------------

* `unsafeFinalise`
- [ ] Result never used

`Database.LSMTree.Monoidal`
---------------------------

* `withTable`
- [ ] Never used
* `new`
- [ ] Never used
* `close`
- [ ] Never used
* `lookups`
- [ ] Never used
* `rangeLookup`
- [ ] Never used
* `withCursor`
- [ ] Never used
* `withCursorAtOffset`
- [ ] Never used
* `newCursor`
- [ ] Never used
* `newCursorAtOffset`
- [ ] Never used
* `closeCursor`
- [ ] Never used
* `readCursor`
- [ ] Never used
* `updates`
- [ ] Never used
* `inserts`
- [ ] Never used
* `deletes`
- [ ] Never used
* `mupserts`
- [ ] Never used
* `createSnapshot`
- [ ] Never used
* `openSnapshot`
- [ ] Never used
* `duplicate`
- [ ] Never used
* `union`
- [ ] Never used
* `unions`
- [ ] Never used
* `remainingUnionDebt`
- [ ] Never used
* `supplyUnionCredits`
- [ ] Never used
* `resolve`
- [ ] Never used

`Database.LSMTree.Normal`
-------------------------

* `withTable`
- [ ] Never used
* `new`
- [ ] Never used
* `close`
- [ ] Never used
* `Bifunctor LookupResult`
- [ ] Never used
* `lookups`
- [ ] Never used
* `Bifunctor (QueryResult k)`
- [ ] Never used
* `rangeLookup`
- [ ] Never used
* `withCursor`
- [ ] Never used
* `withCursorAtOffset`
- [ ] Never used
* `newCursor`
- [ ] Never used
* `newCursorAtOffset`
- [ ] Never used
* `closeCursor`
- [ ] Never used
* `readCursor`
- [ ] Never used
* `toNormalQueryResult`
- [ ] Never used
* `updates`
- [ ] Never used
* `inserts`
- [ ] Never used
* `deletes`
- [ ] Never used
* `retrieveBlobs`
- [ ] Never used
* `createSnapshot`
- [ ] Never used
* `openSnapshot`
- [ ] Never used
* `duplicate`
- [ ] Never used
* `union`
- [ ] Never used
* `unions`
- [ ] Never used
* `remainingUnionDebt`
- [ ] Never used
* `supplyUnionCredits`
- [ ] Never used