Update: action translation + conflict resolution

Parent PRD

Milestone: Action-based Transactions (UserOperation) — see milestone #11. Discussion: #5960. Design spike: #6448.

What to build

Operation::Update has no action translation and still uses a legacy build_manifest arm. The auxiliary-field design is settled in #6843; this issue is the implementation it defers to. Combined vertical slice — translation, apply routing, and conflict migration.

Translate Update per the #6843 decision:

• RewriteRows: RemoveFragments(removed_fragment_ids) + AddFragments(new_fragments, preserved_row_ids) + UpdateDeletionVector per leftover partial delete + InvalidateIndexCoverage / RebindIndexCoverage over modified_field_ids + optional UpdateMergedGenerations.
• RewriteColumns: one ReplaceFragmentColumns per (fragment, field-set) + InvalidateIndexCoverage.
• update_mode is encoded structurally by which actions are emitted. inserted_rows_filter rides on AddFragments as conflict-detection metadata.

Then: route build_manifest through Action::apply and delete the legacy Update arm; implement reads() / writes() / rebase() for the actions involved; add a generator; port the legacy check_update_txn unit tests; extend the differential matrix; fill design-doc §5/§6.

This is the rebase-heaviest operation: rebase must handle affected_rows row-address intersection and inserted_rows_filter bloom-filter intersection on concurrent inserts. Round-trip losslessly.

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

Acceptance criteria

• Update translates to actions for both RewriteRows and RewriteColumns; round-trip test passes
• build_manifest routed through Action::apply; legacy Update arm deleted
• reads/writes/rebase complete for the Update decomposition, including affected_rows and inserted_rows_filter handling in rebase
• Property-based generator added
• Legacy check_update_txn unit tests ported and green on the new resolver
• Differential matrix extended; green for all migrated operations
• Existing update / merge-insert tests still pass
• conflict_resolver.rs unmodified

Blocked by

• #6892 (conflict-resolution framework)
• Depends on ReplaceFragmentColumns from #6842

User stories addressed

• User story 5: single code path for applying operations
• User story 6: single code path for conflict resolution
• User story 9: old operations translated to actions

Start with Operation::Update, build_manifest, and Action::apply, then read the decisions in #6843 and the dependencies #6892 and #6842. Use the legacy check_update_txn tests, the differential matrix, and the design document’s §5/§6 as guides; leave conflict_resolver.rs unchanged. Done means both update modes round-trip through actions, rebase handles affected_rows and inserted_rows_filter, and the listed tests and matrices pass.