Add API verification test suite and documentation

tests/docs/:
- API_FINDINGS.md: Comprehensive migration guide from deprecated to modern API
- test_*.py: 9 executable tests verifying actual runtime behavior
- screenshots/: Visual verification of working examples

tests/conftest.py:
- Add 'docs' and 'demo' to pytest collection paths

Key findings documented:
- Entity uses grid_pos= not pos=
- Scene API: Scene() + activate() replaces createScene/setScene
- scene.children replaces sceneUI()
- scene.on_key replaces keypressScene()
- mcrfpy.current_scene (property) replaces currentScene() (function)
- Timer callback signature: (timer, runtime)
- Opacity animation does NOT work on Frame (documented bug)

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

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

tests/docs/test_entity_api.py

@@ -0,0 +1,30 @@
+#!/usr/bin/env python3
+"""Quick test to verify Entity constructor signature."""
+import mcrfpy
+import sys
+
+scene = mcrfpy.Scene("test")
+scene.activate()
+mcrfpy.step(0.01)
+
+texture = mcrfpy.Texture("assets/kenney_tinydungeon.png", 16, 16)
+grid = mcrfpy.Grid(grid_size=(20, 15), texture=texture, pos=(10, 10), size=(640, 480))
+scene.children.append(grid)
+
+# Test grid_pos vs grid_x/grid_y
+try:
+    e1 = mcrfpy.Entity(grid_pos=(5, 5), texture=texture, sprite_index=85)
+    grid.entities.append(e1)
+    print("grid_pos= WORKS")
+except TypeError as e:
+    print(f"grid_pos= FAILS: {e}")
+
+try:
+    e2 = mcrfpy.Entity(grid_x=7, grid_y=7, texture=texture, sprite_index=85)
+    grid.entities.append(e2)
+    print("grid_x=/grid_y= WORKS")
+except TypeError as e:
+    print(f"grid_x=/grid_y= FAILS: {e}")
+
+print("Entity API test complete")
+sys.exit(0)