Add internal-schema versioning + auto-migration for __manifest

The on-disk shape of `__manifest` is reconciled with the binary via a single
stamp + dispatcher in `db/manifest/migrations.rs`:

- `INTERNAL_MANIFEST_SCHEMA_VERSION = 2` declares the shape this binary writes.
- The on-disk stamp `omnigraph:internal_schema_version` lives in the manifest
  dataset's schema-level metadata (Lance `update_schema_metadata`).
- `migrate_internal_schema(&mut dataset)` walks `match`-arm steps forward from
  the on-disk stamp until it matches the binary, then returns. Idempotent.
- `init_manifest_repo` stamps the current version at creation; the publisher's
  open-for-write path runs pending migrations before reading state. Reads
  stay side-effect-free.
- Forward-version protection: a stamp higher than the binary's known version
  triggers a clear "upgrade omnigraph first" error so an old binary cannot
  clobber a newer schema.

Self-heals existing pre-MR-766 deployments by auto-applying the v1→v2 step:
the `lance-schema:unenforced-primary-key` annotation on `__manifest.object_id`
that engages Lance's row-level CAS at commit time. New repos created via
`init` are stamped at v2 immediately and don't need migration.

Adding a future on-disk shape change is one constant bump, one match arm in
`migrate_internal_schema`, and one test — no new branches in unrelated code
paths. Code outside the migration module never inspects the stamp.

New tests in `manifest/tests.rs`:
- `test_init_stamps_internal_schema_version`
- `test_publish_migrates_pre_stamp_manifest_to_current_version`
- `test_publish_rejects_manifest_stamped_at_future_version`

Docs: `docs/storage.md`, `docs/maintenance.md`, `docs/constants.md` updated
per the AGENTS.md maintenance contract.

docs/constants.md

@@ -8,6 +8,7 @@
 | Run branch prefix | `__run__` | `db/run_registry.rs` |
 | Schema apply lock | `__schema_apply_lock__` | `db/mod.rs` |
 | Manifest publisher retry budget | `PUBLISHER_RETRY_BUDGET = 5` | `db/manifest/publisher.rs` |
+| Internal manifest schema version | `INTERNAL_MANIFEST_SCHEMA_VERSION = 2` | `db/manifest/migrations.rs` |
 | Merge stage batch | `MERGE_STAGE_BATCH_ROWS = 8192` | `exec/merge.rs` |
 | Maintenance concurrency | `OMNIGRAPH_MAINTENANCE_CONCURRENCY=8` | `db/omnigraph/optimize.rs` |
 | Graph index cache size | `8` (LRU) | `runtime_cache.rs` |