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/user/operations/maintenance.md

@@ -6,6 +6,7 @@
 
 - Compacts every node + edge table on `main`, then reindexes them, then **publishes the resulting version to the `__manifest`** so the manifest's recorded version tracks the compacted-and-reindexed state. Reads pin the manifest version, so without this publish the work would be invisible to readers *and* would break the version precondition of the next schema apply / strict update/delete ("stale view … refresh and retry"). The publish advances the graph version (a system-attributed commit) only for tables that actually changed.
 - Rewrites small fragments into fewer large ones; old fragments remain reachable via older versions until `cleanup` runs.
+- **Also compacts the internal system tables** `__manifest` and `_graph_commits` (RFC-013 step 2), which accumulate one fragment per commit and otherwise make every write's metadata scan grow with history. These take a simpler path than data tables: they are not `__manifest`-tracked (readers open them at their latest version), so compaction just advances their version in place — **no manifest publish and no recovery sidecar** (a single atomic Lance commit). They appear in the returned stats under `table_key` `"__manifest"` / `"_graph_commits"`. They are **not yet covered by `cleanup`**, so their version chain still grows until the cleanup half lands (it requires a cleanup-resurrection safeguard first); run `optimize` on a cadence to keep per-write metadata scans flat.
 - **Reindex (index coverage maintenance).** A scalar/FTS/vector index only covers the fragments it was built over. Rows appended after the index was built (e.g. by `load --mode merge`, whose commit does not rebuild an already-existing index) are scanned unindexed, and compaction itself rewrites fragments out of an index's coverage. `optimize` runs Lance's incremental `optimize_indices` after compaction to fold those fragments back in (a delta merge, not a full retrain), restoring full coverage so equality/range/traversal predicates stay index-accelerated. This is why a table with **no compaction work but stale index coverage still commits** a new version under `optimize`. Run `optimize` on a cadence at least as frequent as your freshness window so recently-loaded rows do not linger in the unindexed flat-scan tail.
 - **Create declared-but-missing indexes (the index reconciler).** `@index`/`@key` declares intent; `schema apply` records it but builds nothing, and `load`/`mutate` defer a column that cannot be built yet (a `Vector` column with no trainable vectors). `optimize` materializes any such declared-but-unbuilt index over the compacted layout — so it is the convergence path for an `@index` added after data exists, or a vector index whose embeddings arrived via a later `embed`. A column still not buildable (no vectors yet) is reported on the table's stat as `pending_indexes` (visible in `--json`), not treated as a failure; the next `optimize` retries. So `optimize` is the single operator-facing index reconciler: it compacts, restores coverage, **and** builds declared-but-missing indexes.
 - Each table's compact→reindex→publish serializes with concurrent mutations on the same table. A crash mid-operation is recovered automatically on the next open (both compaction and reindex are content-preserving, so roll-forward is always safe).