DataReplacement: ReplaceFragmentColumns action + translation + conflict resolution

Parent PRD

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

Re-scoped. Originally translation-only. The ReplaceFragmentColumns action that spike #6448 assigned to #6450 was never added to the Action enum, and conflict resolution is now migrated per-operation (framework: #6892). This issue is therefore the full DataReplacement vertical slice.

What to build

Operation::DataReplacement has no action translation and still uses a legacy build_manifest arm. Combined vertical slice — new action, translation, apply routing, conflict migration.

• Define the ReplaceFragmentColumns action (spike §4 A4): payload { fragment_id, field_ids, new_data_files, updated_row_offsets }; implement apply and validate. validate enforces the same-layout-across-fragments rule that Operation::DataReplacement checks today, reading the current per-field file layout from &Manifest — no expected_layout_hash payload field.
• Translate DataReplacement → [ReplaceFragmentColumns*], one per DataReplacementGroup. The replaced field-set is inferred from the swapped DataFile. Translation is apply-time (needs &Manifest). Round-trip losslessly.
• Route build_manifest through Action::apply and delete the legacy DataReplacement arm.
• Implement reads() / writes() / rebase() for ReplaceFragmentColumns.
• Add a property-based generator; port the legacy check_data_replacement_txn unit tests onto the new resolver; extend the differential matrix; fill design-doc §5/§6.

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

Acceptance criteria

• ReplaceFragmentColumns action defined; apply + validate (same-layout rule) implemented
• DataReplacement translated to ReplaceFragmentColumns actions; round-trip test passes
• build_manifest routed through Action::apply; legacy DataReplacement arm deleted
• reads/writes/rebase implemented for ReplaceFragmentColumns
• Property-based generator added
• Legacy check_data_replacement_txn unit tests ported and green on the new resolver
• Differential matrix extended; green for all migrated operations
• Existing data-replacement tests still pass
• conflict_resolver.rs unmodified

Blocked by

• #6892 (conflict-resolution framework)

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
• User story 11: lossless round-trip

Start with the DataReplacement handling, build_manifest, Action::apply, and the legacy check_data_replacement_txn tests described in the issue, then read design spike #6448 sections §4, §5, and §6. Track the conflict-resolution framework in #6892 before beginning. Done means the listed action, translation, routing, resolver behavior, generators, tests, and differential matrix are complete while conflict_resolver.rs remains unchanged.