Remove legacy Operation code paths

Parent PRD

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

Re-scoped. Originally "remove all legacy Operation logic". The build_manifest half is done — 9/13 operations route through Action::apply; the rest land via the per-operation issues below. The conflict-resolution half cannot be sliced per-operation in production (a conflict check is per pair of transactions, so a transition would hit mixed migrated/non-migrated pairs), so it is migrated in a parallel resolver validated by an M×M differential matrix (#6892 + per-operation issues). This issue is now the final cutover.

What to build

Precondition: every operation has been migrated onto the parallel action-level resolver and the differential matrix is green for the full N×N operation set.

• Swap the production commit path from TransactionRebase::check_txn to the action-level resolver (a one-line call-site change against the legacy-shaped adapter from #6892).
• Delete the per-Operation check_*_txn methods and all now-dead code in rust/lance/src/io/commit/conflict_resolver.rs.
• Delete the differential test harness — its legacy oracle is gone. Keep the ported per-operation conflict unit tests; they are the permanent coverage.
• Delete any remaining legacy build_manifest arms and the vestigial Action::conflicts_with.
• cargo fmt --all; cargo clippy --all --tests --benches -- -D warnings clean.

The Operation enum itself remains for deserialization of old transaction files; only its apply/conflict logic is removed.

Acceptance criteria

• Production commit path uses the action-level resolver
• Per-Operation check_*_txn methods removed
• Legacy build_manifest arms removed
• Differential harness removed; ported unit tests retained and green
• All existing tests pass through the action code path
• No regression in transaction processing behavior

Blocked by

• #6892 (conflict-resolution framework)
• #6893, #6894, #6895 (already-translated operations)
• #6896, #6897, #6898, #6842 (Overwrite / Rewrite / Update / DataReplacement)

User stories addressed

• User story 5: single code path for applying operations
• User story 6: single code path for conflict resolution

After #6892 and the listed per-operation migrations are complete, inspect rust/lance/src/io/commit/conflict_resolver.rs and the production commit path. Switch the call site to the action-level resolver, remove the obsolete Operation conflict methods, differential harness, legacy build_manifest arms, and Action::conflicts_with. Retain the ported unit tests, then run the full test suite, cargo fmt --all, and cargo clippy --all --tests --benches -- -D warnings.