feat(09-05): project writable fields through Fill and Validate

- Bind schema fields to model columns at activation and drop protected keys
- Create and update Fill, run BeforeValidate, then Validate before persistence
- Missing Fill or Validate capability and provider errors fail closed

cabana/registry.go

@@ -53,12 +53,16 @@ func compileRegistry(items []controllerRef) (*Registry, error) {
 		if err != nil {
 			return nil, err
 		}
-		byID[id] = &CompiledController{
+		compiled := &CompiledController{
 			PluginID:   item.plugin.ID(),
 			Controller: item.ctl,
 			List:       list,
 			Form:       form,
 		}
+		if err := BindWritableFields(compiled); err != nil {
+			return nil, err
+		}
+		byID[id] = compiled
 	}
 	return &Registry{byID: byID}, nil
 }