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/handlers/packages.go

@@ -553,7 +553,7 @@ func (h *PackageHandler) registerPackage(c *gin.Context, pkgID string, mInfo *Ma
 // and runs schema migrations.
 func (h *PackageHandler) applySchemaAndPermissions(c *gin.Context, pkgID string, mInfo *ManifestInfo, manifest map[string]any, existing *store.PackageRegistration) error {
 	if tables, ok := ParseDBTables(manifest); ok {
-		if err := CreateExtTables(c.Request.Context(), database.DB, database.IsPostgres(), h.stores, pkgID, tables); err != nil {
+		if err := CreateExtTables(c.Request.Context(), database.DB, database.IsPostgres(), h.stores, pkgID, tables, h.capabilities["pgvector"]); err != nil {
 			log.Printf("[ext-db] schema create failed for %s: %v", pkgID, err)
 		}
 	}
@@ -1047,7 +1047,7 @@ func (h *PackageHandler) UpdatePackage(c *gin.Context) {
 	// 6. Additive schema migration
 	var schemaChanges []string
 	if tables, ok := ParseDBTables(manifest); ok {
-		changes, err := MigrateExtTables(ctx, database.DB, database.IsPostgres(), h.stores, pkgID, tables)
+		changes, err := MigrateExtTables(ctx, database.DB, database.IsPostgres(), h.stores, pkgID, tables, h.capabilities["pgvector"])
 		if err != nil {
 			c.JSON(http.StatusInternalServerError, gin.H{"error": "schema migration failed: " + err.Error()})
 			return