Migrate schema operations (Project, Merge) to action-level conflict resolution

Parent PRD

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

What to build

Project and Merge already translate to actions and route build_manifest through Action::apply (done under lance-format/lance#6454). This issue migrates their conflict resolution onto the parallel action-level resolver built in lance-format/lance#6892.

For the actions these operations use (ChangeSchema, AddFields, DropFields, UpdateSchemaMetadata):

• Implement reads() / writes() / rebase().
• Add property-based transaction generators.
• Port the legacy check_project_txn / check_merge_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 (lance-format/lance#6454).

Acceptance criteria

• reads/writes/rebase implemented for ChangeSchema, AddFields, DropFields, UpdateSchemaMetadata
• 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

• lance-format/lance#6892 (conflict-resolution framework)

User stories addressed

• User story 6: single code path for conflict resolution

Start with the conflict-resolution framework in lance-format/lance#6892 and the existing action-level implementations for ChangeSchema, AddFields, DropFields, and UpdateSchemaMetadata. Then inspect the legacy check_project_txn and check_merge_txn tests, the differential matrix, and the design document §§5/6. Done means the generators, resolver behavior, ported tests, matrix, and design rules are complete while conflict_resolver.rs remains unchanged.