fix: address PR #224 review feedback

- NotConnected → NotConnectedWrite in all write-requiring handlers
- NewNotConnectedMsg constructor for custom hints (cmd_features.go)
- Timeout returns ValidationError instead of plain fmt.Errorf
- Standardize RawUnitBackend on model.ID
- Registry panic prefix corrected to 'registry:'
- Full qualified names in AlreadyExists/NotFound messages
- Import/export mapping: distinguish not-found from backend errors
- Published REST: propagate backend errors, include qualified names
- Clarify unhandled statement error message in registry dispatch
- Registry.Validate returns ValidationError instead of plain fmt.Errorf
- Split MockBackend into 13 domain-specific files
- Introduce EntityAccessRuleParams struct for AddEntityAccessRule

Author: retran

mdl/backend/infrastructure.go

@@ -19,7 +19,7 @@ type RenameBackend interface {
 // manipulate raw BSON (e.g. widget patching, alter page/workflow).
 type RawUnitBackend interface {
 	GetRawUnit(id model.ID) (map[string]any, error)
-	GetRawUnitBytes(id string) ([]byte, error)
+	GetRawUnitBytes(id model.ID) ([]byte, error)
 	ListRawUnitsByType(typePrefix string) ([]*mpr.RawUnit, error)
 	ListRawUnits(objectType string) ([]*mpr.RawUnitInfo, error)
 	GetRawUnitByName(objectType, qualifiedName string) (*mpr.RawUnitInfo, error)