Implement Scene subclass on_key callback support

Scene subclasses can now define on_key(self, key, state) methods that
receive keyboard events, matching the existing on_enter, on_exit, and
update lifecycle callbacks.

Changes:
- Rename call_on_keypress to call_on_key (consistent naming with property)
- Add triggerKeyEvent helper in McRFPy_API
- Call triggerKeyEvent from GameEngine when key_callable is not set
- Fix condition to check key_callable.isNone() (not just pointer existence)
- Handle both bound methods and instance-assigned callables

Usage:
    class GameScene(mcrfpy.Scene):
        def on_key(self, key, state):
            if key == "Escape" and state == "end":
                quit_game()

Property assignment (scene.on_key = callable) still works and takes
precedence when key_callable is set via the property setter.

Includes comprehensive test: tests/unit/scene_subclass_on_key_test.py

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/scene_subclass_on_key_test.py

@@ -0,0 +1,84 @@
+"""Test Scene subclass on_key method callback
+
+Verifies that:
+1. Subclass on_key method is called for keyboard events
+2. Property assignment (scene.on_key = callable) still works
+3. Property assignment on subclass overrides the method
+"""
+import mcrfpy
+from mcrfpy import automation
+import sys
+
+# Test state
+tests_passed = 0
+tests_failed = 0
+
+def test_subclass_method():
+    """Test that subclass on_key method receives keyboard events"""
+    global tests_passed, tests_failed
+    events = []
+
+    class TestScene(mcrfpy.Scene):
+        def on_key(self, key, state):
+            events.append((key, state))
+
+    ts = TestScene('test_method')
+    ts.activate()
+    automation.keyDown('a')
+    automation.keyUp('a')
+
+    if len(events) >= 2:
+        print("PASS: test_subclass_method")
+        tests_passed += 1
+    else:
+        print(f"FAIL: test_subclass_method - got {events}")
+        tests_failed += 1
+
+def test_property_handler():
+    """Test that property assignment works"""
+    global tests_passed, tests_failed
+    events = []
+
+    scene = mcrfpy.Scene('test_property')
+    scene.on_key = lambda k, s: events.append((k, s))
+    scene.activate()
+    automation.keyDown('b')
+    automation.keyUp('b')
+
+    if len(events) >= 2:
+        print("PASS: test_property_handler")
+        tests_passed += 1
+    else:
+        print(f"FAIL: test_property_handler - got {events}")
+        tests_failed += 1
+
+def test_property_overrides_method():
+    """Test that property assignment on subclass overrides the method"""
+    global tests_passed, tests_failed
+    method_events = []
+    property_events = []
+
+    class TestScene(mcrfpy.Scene):
+        def on_key(self, key, state):
+            method_events.append((key, state))
+
+    ts = TestScene('test_override')
+    ts.activate()
+    ts.on_key = lambda k, s: property_events.append((k, s))
+    automation.keyDown('c')
+    automation.keyUp('c')
+
+    if len(property_events) >= 2 and len(method_events) == 0:
+        print("PASS: test_property_overrides_method")
+        tests_passed += 1
+    else:
+        print(f"FAIL: test_property_overrides_method - method={method_events}, property={property_events}")
+        tests_failed += 1
+
+# Run tests
+test_subclass_method()
+test_property_handler()
+test_property_overrides_method()
+
+print(f"\nResults: {tests_passed} passed, {tests_failed} failed")
+sys.exit(0 if tests_failed == 0 else 1)