staging: op-kind-aware drift check at commit_all entry

Closes the bug class "Lance internal conflict surfaces as 500 instead
of 409" for in-process concurrent strict-op writers on the same row.

Pre-fix: in `MutationStaging::commit_all`, after queue acquisition, the
staged Lance transaction (built against V0) was handed straight to
`commit_staged`. When Lance HEAD has advanced past V0 (because the
queue's prior winner already published), Lance's transaction conflict
resolver fires `RetryableCommitConflict` for Update vs Update on the
same row, which wraps as `OmniError::Lance(<string>)` and the API maps
it to HTTP 500. Users see "internal server error" instead of a clean
retryable conflict.

Fix: track the strictest `MutationOpKind` per touched table on
`MutationStaging` and propagate through `StagedMutation`. In
`commit_all`'s recapture loop, before each `commit_staged`, fail-fast
with `OmniError::manifest_expected_version_mismatch` (→ HTTP 409
ExpectedVersionMismatch) for tables whose tracked op_kind has
`strict_pre_stage_version_check() == true` (Update/Delete/SchemaRewrite)
when the staged dataset's version doesn't match the fresh manifest pin
under the queue. Insert/Merge tables skip the check — concurrent
inserts on disjoint keys legitimately coexist via Lance's auto-rebase,
so the check would over-reject the existing same-key insert path.

Threading: `ensure_path` now takes `op_kind` and stores it in a new
`op_kinds: HashMap<String, MutationOpKind>` on `MutationStaging`, with
strictness-upgrade semantics so mixed insert+update on the same table
still fires the strict check at commit time. `StagedMutation` carries
`op_kinds` through to `commit_all`.

Pinned by `change_concurrent_updates_same_key_serialize_via_publisher_cas`
in `crates/omnigraph-server/tests/server.rs` (added in the previous
commit). All Phase 2 sentinels still pass:
change_concurrent_inserts_same_key_serialize_without_409,
change_conflict_returns_manifest_conflict_409,
branch_merge_conflict_response_includes_structured_conflicts.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/omnigraph/src/loader/mod.rs

@@ -383,6 +383,7 @@ async fn load_jsonl_reader<R: BufRead>(
                 full_path,
                 table_branch,
                 expected_version,
+                load_op_kind,
             );
             let schema = batch.schema();
             staging.append_batch(&table_key, schema, pending_mode, batch)?;
@@ -504,6 +505,7 @@ async fn load_jsonl_reader<R: BufRead>(
                 full_path,
                 table_branch,
                 expected_version,
+                load_op_kind,
             );
             let schema = batch.schema();
             staging.append_batch(&table_key, schema, pending_mode, batch)?;