Overwrite: action translation + conflict resolution

Parent PRD

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

What to build

Operation::Overwrite has no action translation and still uses a legacy build_manifest arm. This is a combined vertical slice — translation, apply routing, and conflict migration in one operation.

• Translate Overwrite → [RemoveFragments(AllCurrent), ChangeSchema, AddFields?, AddFragments, UpdateConfig?, AddBases?]. Requires the RemoveFragments Selector { Ids, AllCurrent } form (spike #6448 Q3) if not yet present. Round-trip losslessly.
• Route build_manifest through Action::apply and delete the legacy Overwrite arm.
• Implement reads() / writes() / rebase() for any action in the list that still lacks it.
• Add a property-based generator; port the legacy check_overwrite_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

• Overwrite translates to actions; round-trip test passes
• build_manifest routed through Action::apply; legacy Overwrite arm deleted
• reads/writes/rebase complete for all actions in the Overwrite decomposition
• Property-based generator added
• Legacy check_overwrite_txn unit tests ported and green on the new resolver
• Differential matrix extended; green for all migrated operations
• Existing overwrite 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

Start with blocked issue #6892, then inspect Operation::Overwrite, build_manifest, and Action::apply to understand the existing translation and routing. Read the legacy check_overwrite_txn tests and the design document sections 5 and 6 before adding the generator and differential coverage. Done means all acceptance criteria pass while conflict_resolver.rs remains unchanged.