Migrate metadata operations to action-level conflict resolution

Parent PRD

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

What to build

UpdateConfig, UpdateBases, UpdateMemWalState, and ReserveFragments already translate to actions and route build_manifest through Action::apply (done under #6454). This issue migrates their conflict resolution onto the parallel action-level resolver built in #6892.

For the actions these operations use (UpdateConfig, AddBases, UpdateMergedGenerations, ReserveFragmentIds):

• Implement reads() / writes() / rebase().
• Add property-based transaction generators.
• Port the legacy check_update_config_txn / check_add_bases_txn / check_update_mem_wal_state_txn / check_reserve_fragments_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.

Also completes UpdateConfig's translation to cover schema- and field-metadata updates: actions_from_operation currently returns None for these, so the schema/field-metadata conflicts that Operation::modifies_same_metadata detects would be silently lost. Split them onto the UpdateSchemaMetadata action per spike §4.

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 UpdateConfig, AddBases, UpdateMergedGenerations, ReserveFragmentIds
• UpdateConfig translation covers schema/field-metadata (no longer returns None)
• 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, the relevant action implementations, actions_from_operation, and the legacy check_update_config_txn, check_add_bases_txn, check_update_mem_wal_state_txn, and check_reserve_fragments_txn tests; keep conflict_resolver.rs unchanged. Compare the design spike and update the differential matrix and design doc §5/§6, with all migrated operations and property-based tests green when done.