Feat v0.8.3 vector column type (#70)

Add vector(N) column type to db_tables manifests with three-tier
progressive enhancement: native pgvector on Postgres, JSONB fallback
without pgvector, TEXT fallback on SQLite. New db.query_similar()
Starlark builtin with dual-path dispatch.

- parseVectorDim validates 1..4096 dimensions
- mapColType gains hasPgvector parameter for tier selection
- HNSW index auto-created on pgvector backends
- starlarkToGoValue extended with list→JSON serialization
- cosineDistance helper for Go-side fallback computation
- ExtensionHandler gains SetCapabilities for install-time DDL
- Roadmap updated: v0.8.4 docs refresh + surface sizing fix

13 new tests (5 schema + 8 db module), all passing with -race.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -2,6 +2,40 @@
 
 All notable changes to Armature are documented here.
 
+## v0.8.3 — Vector Column Type
+
+Extensions can now declare vector columns and perform similarity search.
+Three-tier progressive enhancement: native pgvector on Postgres, JSONB
+fallback without pgvector, TEXT fallback on SQLite.
+
+**Manifest: `db_tables` vector columns**
+
+- Declare `"vector(N)"` as a column type (N = dimension, 1–4096).
+- On Postgres with pgvector: native `vector(N)` type with auto-created
+  HNSW index (`vector_cosine_ops`).
+- On Postgres without pgvector: `JSONB` column.
+- On SQLite: `TEXT` column (JSON-encoded float arrays).
+
+**Starlark API**
+
+- `db.query_similar(table, column, vector=[], limit=10, filters={}, metric="cosine")`
+  — returns rows ordered by ascending cosine distance with injected `_distance` key.
+- `db.insert()` now accepts list values (serialized as JSON strings) for
+  vector column storage.
+
+**Internal**
+
+- Modified: `handlers/ext_db_schema.go` — `parseVectorDim`, `mapColType`
+  gains `hasPgvector` parameter, HNSW index creation for vector columns.
+- Modified: `sandbox/db_module.go` — `HasPgvector` in `DBModuleConfig`,
+  list support in `starlarkToGoValue`, `dbQuerySimilar` with pgvector and
+  fallback paths, `cosineDistance` helper.
+- Modified: `sandbox/runner.go` — wire `HasPgvector` from capabilities.
+- Modified: `handlers/extensions.go` — `SetCapabilities` on `ExtensionHandler`.
+- Modified: `server/main.go` — wire capabilities to extension handler.
+- Updated: `docs/STARLARK-REFERENCE.md` — vector similarity section.
+- New tests: 5 schema tests + 8 db module tests (13 total).
+
 ## v0.8.2 — Capability Negotiation
 
 Extensions declare environment requirements in their manifest. The kernel