[blkstorage] Extract only the header when only the header is read

Part of the umbrella #167.

Two paths want nothing but a block header, and both parse the entire serialized block to get it.
`extractSerializedBlockInfo` (`common/ledger/blkstorage/block_serialization.go:56`) decodes every
envelope, computes a transaction ID per envelope, and decodes all metadata entries:

- `blockfileMgr.retrieveBlockHeaderByNumber` (`blockfile_mgr.go:604`) reads `info.blockHeader` and
drops the rest. It runs at every store open, on the last persisted block, to build the initial
`BlockchainInfo` (`blockfile_mgr.go:159`).
- `retrieveFirstBlockNumFromFile` (`blockfile_helper.go:127`) reads `blockInfo.blockHeader.Number`
and drops the rest. `binarySearchFileNumForBlock` calls it once per probe, so a rollback pays it
O(log n) times over the block files.

Neither is on a steady-state hot path, so this is smaller than #165 and #182.
It is worth doing anyway because the fix is a few lines and the current cost is unbounded in block
size: opening a store that last wrote a 10,000-transaction block unmarshals 10,000 envelopes to read
three header fields.

Proposal: add a header-only extractor that stops after `extractHeader`, and use it in both places.
The Fabric-X Orderer's fork of this package already carries exactly this as
`extractSerializedBlockHeader`, used in the same two call sites.

Note that #182 also touches `extractSerializedBlockInfo`; if that lands
first this becomes smaller still, but the two are independent — a header-only path skips the
metadata decode and the envelope decode as well, not just the transaction IDs.

Start in common/ledger/blkstorage/block_serialization.go at extractSerializedBlockInfo and extractHeader, then compare the existing extractSerializedBlockHeader implementation in the Fabric-X Orderer fork. Trace blockfileMgr.go:604 and blockfile_helper.go:127, where only the header is needed. Done means both call sites use the header-only path without decoding the rest of the serialized block.

Written by the indexing model from the issue text.