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>

server/sandbox/runner.go

@@ -425,10 +425,11 @@ func (r *Runner) buildModulesWithLibCtx(ctx context.Context, packageID string, m
 	// Wire db module at the highest granted level.
 	if dbLevel > 0 && r.db != nil {
 		modules["db"] = BuildDBModule(ctx, DBModuleConfig{
-			PackageID:  packageID,
-			CanWrite:   dbLevel == 2,
-			DB:         r.db,
-			IsPostgres: r.dbPostgres,
+			PackageID:   packageID,
+			CanWrite:    dbLevel == 2,
+			DB:          r.db,
+			IsPostgres:  r.dbPostgres,
+			HasPgvector: r.capabilities["pgvector"],
 		})
 	}