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
- Move mpr type extraction TODO to project tracker

Author: retran

mdl/backend/doc.go

@@ -4,7 +4,7 @@
 // executor from concrete storage (e.g. .mpr files). Each interface
 // groups related read/write operations by domain concept.
 //
-// TODO: Several method signatures currently reference types from sdk/mpr
+// Several method signatures currently reference types from sdk/mpr
 // (e.g. NavigationDocument, FolderInfo, ImageCollection, JsonStructure,
 // JavaAction, EntityMemberAccess, RenameHit). These should eventually be
 // extracted into a shared types package to remove the mpr dependency.

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)