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/main.go

@@ -809,6 +809,7 @@ func main() {
 
 			// Extensions (admin)
 			extAdm := handlers.NewExtensionHandler(stores)
+			extAdm.SetCapabilities(detectedCaps)
 			admin.GET("/extensions", extAdm.AdminListExtensions)
 			admin.POST("/extensions", extAdm.AdminInstallExtension)
 			admin.PUT("/extensions/:id", extAdm.AdminUpdateExtension)