canvas: overhaul editor with undo/redo, selection model, alignment, and validation

Selection model:
- Single select, multi-select (Shift+click), box select (Shift+drag)
- Selection info panel showing position, size, name, description

Undo/redo:
- Command stack with MoveCommand and PropertyCommand
- Undo/redo buttons with max depth of 50

Commands:
- Align left/right/top/bottom
- Distribute horizontally/vertically
- Bring forward/send backward (z-order)
- Copy/paste/duplicate

Validation before save:
- Duplicate name detection
- Broken connection checks
- Orphaned item detection

Persistence:
- XML format deprecated with warning, auto-migrate to YAML

Author: jirhiker

pychron/canvas/canvas_editor.py

@@ -140,10 +140,15 @@ def undo(self) -> None:
 class UndoStack(HasTraits):
     """Undo/redo command stack."""
 
-    _undo_stack: List[Command] = []
-    _redo_stack: List[Command] = []
+    _undo_stack: list[Command]
+    _redo_stack: list[Command]
     max_depth = Int(50)
 
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self._undo_stack = []
+        self._redo_stack = []
+
     def push(self, command: Command) -> None:
         if command._done:
             return