Action-level conflict resolution: framework + Append/Delete tracer bullet

Parent PRD

Milestone: Action-based Transactions (UserOperation) — see milestone #11 for the full PRD.
Discussion: #5960. Conflict-resolution design spike: #6448.

Background

Spike #6448 produced two outputs: (a) the action catalog and (b) the action trait shape — a five-method contract (reads/writes/validate/rebase/apply) plus a driver loop. Output (a) was threaded into #6450–#6453; output (b) was never assigned to an implementation issue. As a result, build_manifest was migrated to Action::apply (9/13 ops, done under #6454), but conflict resolution still runs entirely through the legacy per-Operation check_*_txn matrix in rust/lance/src/io/commit/conflict_resolver.rs. The shipped Action::conflicts_with -> bool is the pre-spike shape and has no production callers.

Conflict resolution cannot be migrated one operation at a time in production: a conflict check is per pair of transactions, so during a transition a commit can hit a mixed (migrated, non-migrated) pair. The migration is therefore a parallel/shadow implementation, validated by a differential test matrix over the set of already-migrated operations (M×M), cutting over only once M = N.

This issue builds that framework and proves it end-to-end on Append + Delete.

What to build

• Land the spike design doc at rust/lance-table/design/action_based_conflict_resolution.md (skeleton + #6448's resolved open questions; the §5/§6 conflict matrix is filled incrementally by the per-operation issues).
• Implement the spike §7 trait on Action: reads() -> ManifestMask, writes() -> ManifestMask, async rebase(&mut self, current, concurrent); generalize validate across all actions. Define ManifestMask, TxnContext, ActionOutput.
• Implement the transaction driver commit loop (mask-gated rebase, spike §7).
• Implement a parallel conflict-resolution entry point behind a legacy-shaped adapter: same (other_transaction, other_version) -> Result<()> signature and rebased-transaction output as TransactionRebase::check_txn, so the eventual cutover is a one-line call-site swap. Not wired into the production commit path.
• Implement reads/writes/rebase for the actions Append + Delete use: AddFragments, RemoveFragments, UpdateDeletionVector. Remaining actions get explicit unimplemented stubs, filled by their per-operation issue.
• Handle Restore/Clone as top-level non-action transaction kinds in the driver.
• Build the differential test harness + property-based transaction generator infrastructure; generators for Append and Delete.
• Port the legacy Append/Delete conflict unit tests onto the new resolver; feed the same fixtures through the differential harness so unit-test expectations are cross-checked against the legacy oracle.

conflict_resolver.rs must not be modified — it remains the production path and the differential oracle until the final cutover (#6454).

Acceptance criteria

• Design doc landed in-tree
• reads/writes/validate/rebase/apply trait + ManifestMask/TxnContext/ActionOutput defined
• Driver commit loop implemented
• Parallel resolver entry point behind a legacy-shaped adapter
• Differential harness + property-based generators for Append/Delete
• M×M differential tests green for {Append, Delete}
• Legacy Append/Delete conflict unit tests ported and green on the new resolver
• conflict_resolver.rs unmodified

Blocked by

None — startable immediately.

Blocks

Per-operation conflict-resolution migration, then the cutover:

• #6893 — metadata operations (UpdateConfig, UpdateBases, UpdateMemWalState, ReserveFragments)
• #6894 — schema operations (Project, Merge)
• #6895 — CreateIndex
• #6896 — Overwrite (translation + conflict)
• #6897 — Rewrite (translation + conflict)
• #6898 — Update (translation + conflict)
• #6842 — DataReplacement (translation + conflict)
• #6454 — final cutover

User stories addressed

• User story 6: single code path for conflict resolution

Start with rust/lance-table/design/action_based_conflict_resolution.md and the §7 design from #6448, then inspect the existing transaction and action code around the conflict-resolution path. Build the parallel resolver without modifying rust/lance/src/io/commit/conflict_resolver.rs, and use the specified Append/Delete fixtures, generators, and differential tests as the completion check.