Rewrite: RewriteFragments action + translation + conflict resolution

Parent PRD

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

What to build

Operation::Rewrite has no action translation and still uses a legacy build_manifest arm. The RewriteFragments action it needs was specified by spike #6448 (§4 A5) but never added to the Action enum. This is a combined vertical slice — new action, translation, apply routing, and conflict migration.

• Define the RewriteFragments action: payload { old_fragment_ids, new_fragments, preserves_row_ids }; implement apply and validate (asserts data equivalence so index coverage can be salvaged).
• Translate Rewrite → one RewriteFragments per rewrite group, plus: with stable row IDs, RebindIndexCoverage + optional RegisterFragReuse; without stable row IDs, one RewriteIndex per rewritten index. Round-trip losslessly.
• Route build_manifest through Action::apply and delete the legacy Rewrite arm.
• Implement reads() / writes() / rebase() for RewriteFragments and any other action in the decomposition still lacking it.
• Add a property-based generator; port the legacy check_rewrite_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

• RewriteFragments action defined; apply + validate implemented
• Rewrite translates to actions; round-trip test passes
• build_manifest routed through Action::apply; legacy Rewrite arm deleted
• reads/writes/rebase complete for the Rewrite decomposition
• Property-based generator added
• Legacy check_rewrite_txn unit tests ported and green on the new resolver
• Differential matrix extended; green for all migrated operations
• Existing compaction 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

This work is blocked by #6892 and spans the Rewrite action, build_manifest, Action::apply, the legacy check_rewrite_txn tests, the differential matrix, and design-doc §§5/6. Start by reading the Rewrite decomposition and spike #6448 §4 A5; do not modify conflict_resolver.rs. Done means the listed acceptance checks pass, including round-trip and property-based tests, while existing compaction tests remain green.