Remove legacy string enum comparisons from InputState/Key/MouseButton, closes #306

Removed custom __eq__/__ne__ that allowed comparing enums to legacy string
names (e.g., Key.ESCAPE == "Escape"). Removed _legacy_names dicts and
to_legacy_string() functions. Kept from_legacy_string() in PyKey.cpp as
it's used by C++ event dispatch. Updated ~50 Python test/demo/cookbook
files to use enum members instead of string comparisons. Also updates
grid.position -> grid.pos in files that had both types of changes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/docs/test_quickstart_simple_scene.py

@@ -35,7 +35,7 @@ scene.children.append(grid)
 
 # Add keyboard controls using modern API
 def move_around(key, state):
-    if state == "start":
+    if state == mcrfpy.InputState.PRESSED:
         print(f"You pressed {key}")
 
 scene.on_key = move_around