mr-686: bundle PR 0/1a/1b foundation + PR 2 catalog/schema_source ArcSwap

Bundles the working-tree state from the prior session (PR 0 bench harness,
PR 1a audit_actor_id removal, PR 1b WriteQueueManager + writer integration)
together with the first half of PR 2's interior-mutability foundation
(catalog and schema_source wrapped in Arc<ArcSwap<...>>). The two streams
intermix in 7 of the same files, so splitting via git add -p was
impractical. Subsequent PR 2 steps land as separate atomic commits.

PR 0 — server-level concurrent /change bench harness
  - crates/omnigraph-server/examples/bench_concurrent_http.rs (new)
  - .context/bench-results/{baseline-main,after-pr1}/ (gitignored)

PR 1a — drop the audit_actor_id field, thread per-call
  - removed Omnigraph::audit_actor_id and the swap-restore patterns in
    mutation.rs, merge.rs, loader/mod.rs
  - actor_id: Option<&str> threaded through MutationStaging::finalize,
    mutate_with_current_actor, ingest_with_current_actor,
    branch_merge_impl, branch_merge_on_current_target,
    commit_prepared_updates*, record_merge_commit,
    commit_updates_on_branch_with_expected
  - apply_schema and ensure_indices_for_branch pass None (system-attributed)

PR 1b — per-(table_key, branch) write queue + revalidation + sidecar
  - new crates/omnigraph/src/db/write_queue.rs with WriteQueueManager,
    acquire/acquire_many, sorted+deduped acquisition; 6 unit tests
  - Arc<WriteQueueManager> field on Omnigraph + db.write_queue() accessor
  - MutationStaging::finalize split into stage_all (Phase A, no queue)
    and StagedMutation::commit_all (Phase B, acquire_many + revalidate
    pins + sidecar + commit_staged); guards held across publisher
  - delete-only mutations now emit recovery sidecars; revalidation
    extended to inline_committed tables
  - branch_merge_on_current_target, apply_schema_with_lock, and
    ensure_indices_for_branch acquire per-table queues for their
    touched tables

PR 2 Step B (partial) — catalog and schema_source via ArcSwap
  - catalog: Catalog -> Arc<ArcSwap<Catalog>>
  - schema_source: String -> Arc<ArcSwap<String>>
  - public accessors return Arc<Catalog> / Arc<String>; readers bind
    locally where the borrow has to outlive an expression
  - new pub(crate) store_catalog / store_schema_source helpers replace
    the field assignments in apply_schema and reload_schema_if_source_changed
  - 117 tests across lifecycle/end_to_end/branching/runs pass; engine
    lib + workspace compile clean

Coordinator wrap (Mutex) and the &mut self -> &self engine API
conversion follow in subsequent commits.

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

AGENTS.md

@@ -236,5 +236,6 @@ Rules:
 4. **Re-verify before recommending.** If you cite a flag, env var, endpoint, or constant to the user or in code, grep for it in source first. Memory and docs go stale; the code is authoritative.
 5. **Keep AGENTS.md a map, not an encyclopedia.** New deep content goes into `docs/`. Add an entry to "Where to find each topic" instead of pasting prose into this file. The "Always-on rules" section is the exception — it's for invariants that should always be in scope.
 6. **Re-read on schema/query/IR changes.** Edits to `schema.pest`, `query.pest`, `ir/lower.rs`, `query/typecheck.rs`, or `query/lint.rs` should trigger a re-read of [docs/schema-language.md](docs/schema-language.md), [docs/query-language.md](docs/query-language.md), and [docs/execution.md](docs/execution.md) to confirm they still describe reality.
+7. **Always make smaller commits.** Each commit does one thing, compiles, and passes tests; mechanical refactors land separately from the behavior changes they enable.
 
 CI check: `scripts/check-agents-md.sh` verifies that every `docs/*.md` link in this file resolves and that every doc in the canonical set is linked. Run it locally before opening a PR if you've moved or renamed docs.