Update test demos for new Python API and entity system

- Update all text input demos to use new Entity constructor signature
- Fix pathfinding showcase to work with new entity position handling
- Remove entity_waypoints tracking in favor of simplified movement
- Delete obsolete exhaustive_api_demo.py (superseded by newer demos)
- Adjust entity creation calls to match Entity((x, y), texture, sprite_index) pattern

All demos now properly demonstrate the updated API while maintaining their
original functionality for showcasing engine features.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

tests/demos/simple_text_input.py

@@ -28,11 +28,11 @@ class TextInput:
         # Label
         if self.label:
             self.label_caption = mcrfpy.Caption(self.label, self.x, self.y - 20)
-            self.label_caption.color = (255, 255, 255, 255)
+            self.label_caption.fill_color = (255, 255, 255, 255)
         
         # Text display
         self.text_caption = mcrfpy.Caption("", self.x + 4, self.y + 4)
-        self.text_caption.color = (0, 0, 0, 255)
+        self.text_caption.fill_color = (0, 0, 0, 255)
         
         # Cursor (a simple vertical line using a frame)
         self.cursor = mcrfpy.Frame(self.x + 4, self.y + 4, 2, 16)
@@ -176,7 +176,7 @@ def create_scene():
     
     # Title
     title = mcrfpy.Caption("Text Input Widget Demo", 10, 10)
-    title.color = (255, 255, 255, 255)
+    title.fill_color = (255, 255, 255, 255)
     scene.append(title)
     
     # Create input fields
@@ -194,7 +194,7 @@ def create_scene():
     
     # Status text
     status = mcrfpy.Caption("Click to focus, type to enter text", 50, 280)
-    status.color = (200, 200, 200, 255)
+    status.fill_color = (200, 200, 200, 255)
     scene.append(status)
     
     # Keyboard handler