docs: RFC-013 step 2 internal-table compaction landed

- invariants.md: close the compaction half of the read-path-rederivation known
  gap (optimize now compacts the internal tables; cleanup half still deferred).
- maintenance.md: optimize covers __manifest/_graph_commits (no publish, no
  sidecar); not yet in cleanup.
- rfc-013 §9: split step 2 into 2a (compaction, landed) and 2b (cleanup + Q8
  watermark, deferred — debated; MTT-overlap + hot-path liability).
- testing.md: the internal-table LOCK is now green every-PR.

docs/dev/testing.md

@@ -26,7 +26,7 @@ The engine's `tests/` is the principal coverage surface; most graph-shaped behav
 | `forbidden_apis.rs` | Defense-in-depth source-walk guard: engine code (`exec/`, `db/omnigraph/`, `loader/`, `changes/`) must not reach around the sealed storage trait to Lance inline-commit APIs, nor open datasets directly (`Dataset::open` / `DatasetBuilder::from_uri`/`from_namespace`) — reads route through `Snapshot::open` and the held-handle cache; `// forbidden-api-allow: <reason>` sentinel exempts reviewed lines |
 | `lance_surface_guards.rs` | Pins the Lance API surfaces omnigraph depends on (named runtime + compile-only guards; see [lance.md](lance.md)) — the first smoke check on any Lance version bump; e.g. `compact_files_still_fails_on_blob_columns` turns red when the upstream blob-compaction fix lands |
 | `warm_read_cost.rs` | Cost-budget tests for the warm read path (query-latency work), measured at the object-store boundary with Lance `IOTracker` (the LanceDB IO-counted pattern): a warm same-branch read does 0 manifest opens, 0 commit-graph opens, 1 version probe, validates the schema once (Fix 1 / finding A / Fix 2 at commit-history depth); stale same-branch reads perform exactly 2 probes and refresh manifest-only; recreated non-main branches with the same Lance version refresh by incarnation; recreated branch-owned table handles are distinguished by table e_tag or refresh-time cache clearing; recreated traversal topology is protected by synthetic snapshot-id incarnation or refresh-time cache clearing; a warm *repeat* read does 0 table opens via the held-handle cache and a write re-opens only the changed table at its new version/e_tag (Fix 3/6A). See "Cost-budget tests" below |
-| `write_cost.rs` | Cost-budget tests for the WRITE path (RFC-013), the latency twin of `warm_read_cost.rs` on the **shared `helpers::cost` harness** (`measure`/`IoCounts`/`assert_flat`/`local_graph`). Runs on **local FS**; gates the **internal-table** term (`__manifest`/`_graph_commits` scans flat in commit-history depth — the RED `internal_table_scans_are_flat_in_history` LOCK, `#[ignore]`'d until internal-table compaction lands) plus green every-PR guards (single-insert `data_writes` bounded, a per-write read-op ceiling that fails the moment a round-trip is added, and a `measure_with_staged` fitness assert that a keyed insert routes through `stage_merge_insert` once with no `stage_append`/vector-index build). The **data-table opener** term is S3-only — see `write_cost_s3.rs` and the backend-split note in "Cost-budget tests" below |
+| `write_cost.rs` | Cost-budget tests for the WRITE path (RFC-013), the latency twin of `warm_read_cost.rs` on the **shared `helpers::cost` harness** (`measure`/`IoCounts`/`assert_flat`/`local_graph`). Runs on **local FS**; gates the **internal-table** term (`__manifest`/`_graph_commits` scans flat in commit-history depth — `internal_table_scans_are_flat_in_history`, now **green every-PR** since RFC-013 step 2 brought the internal tables into `optimize`; the test compacts at each depth before measuring) plus green every-PR guards (single-insert `data_writes` bounded, a per-write read-op ceiling that fails the moment a round-trip is added, and a `measure_with_staged` fitness assert that a keyed insert routes through `stage_merge_insert` once with no `stage_append`/vector-index build). The **data-table opener** term is S3-only — see `write_cost_s3.rs` and the backend-split note in "Cost-budget tests" below |
 | `helpers/cost.rs` | The shared cost-budget harness (not a test): `IoCounts`/`StagedCounts` (counts by table class), `measure`/`measure_with_staged` (the one place the `with_query_io_probes` + `MergeWriteProbes` task-local + `IOTracker` wiring lives), `assert_flat(curve, select, slack, what)`, and store-agnostic `local_graph`/`s3_graph` fixtures. `warm_read_cost.rs`, `write_cost.rs`, and `write_cost_s3.rs` all consume it so a cost test body is written once and reads in one vocabulary |
 | `lifecycle.rs` | Graph lifecycle, schema state |
 | `point_in_time.rs` | Snapshots, time travel (`snapshot_at_version`, `entity_at`) |