Migrate CreateIndex to action-level conflict resolution

Parent PRD

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

What to build

CreateIndex already translates to actions and routes build_manifest through Action::apply (done under #6454). This issue migrates its conflict resolution onto the parallel action-level resolver built in #6892.

For the index actions (AddIndex, RemoveIndex, and the coverage actions InvalidateIndexCoverage / RebindIndexCoverage / RewriteIndex / RegisterFragReuse as far as they participate in CreateIndex conflicts):

• Implement reads() / writes() / rebase().
• Implement criterion-based index-coverage overlap (spike §3.1): a criterion path — e.g. "all indices over field X" — must conflict-overlap any concrete path that satisfies the criterion, so a concurrent AddFragments or column update is detected correctly.
• Add property-based transaction generators.
• Port the legacy check_create_index_txn unit tests onto the new resolver.
• Extend the differential matrix by these rows/columns; it must stay green for all operations migrated so far.
• Fill the corresponding §5/§6 rules in the design doc.

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

Acceptance criteria

• reads/writes/rebase implemented for the index actions
• Criterion ∩ concrete-path overlap (spike §3.1) implemented and tested
• Property-based generators added
• Legacy conflict unit tests ported and green on the new resolver
• Differential matrix extended; green for all migrated operations
• Design doc §5/§6 updated
• conflict_resolver.rs unmodified

Blocked by

• #6892 (conflict-resolution framework)

User stories addressed

• User story 6: single code path for conflict resolution

Start with the conflict-resolution framework in #6892 and the existing CreateIndex action translation from #6454; keep conflict_resolver.rs unchanged. Review the legacy check_create_index_txn tests, the differential matrix, and design spike §3.1 before implementing the listed action interfaces and generators. Done means all acceptance criteria are met, including green migrated-operation comparisons and updated design sections §5/§6.