feat: report ATL problems only for used transforms; animate moving images

- Unsupported ATL constructs in a transform now block the import only when the
  transform is used (at clause, another transform, even before its definition).
  Stock screens.rpy transforms for unused screens no longer fail games
- image blocks with one picture plus linear/ease/parallel/repeat keep their
  animation (image-level ATL); the renderer also starts it
- Python: skip @gui.variant/@renpy.variant functions (inactive variants), treat
  @renpy.pure as transparent, read renpy.store.x as x
- Accept renpy.transition, end_replay, block_rollback and music set_pause/set_pan
  as presentation-only no-ops, and implement them in the player (set_pause and
  set_pan were allowed by the converter but missing at runtime)
- Skip the 0x52-URM add-on directory (in-game console / save editor) when
  parsing scripts and media
- Tests for lazy ATL warnings, moving images and variants

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

backend/tests/test_compatibility.py

@@ -102,9 +102,14 @@ def test_renpy_atl_animation_becomes_a_timeline_not_a_static_pose():
 def test_renpy_atl_constructs_without_a_runtime_are_still_rejected():
     for body in ("function slide", "time 1.0:\n        xalign 1.0", "matrixcolor TintMatrix('#f00')",
                  "contains:\n        'a.png'"):
-        data = script(f'transform odd:\n    {body}\nlabel start:\n    "Hi"')
+        unused = script(f'transform odd:\n    {body}\nlabel start:\n    "Hi"')
+        require_supported_scripts(unused)  # nothing shows it, so it cannot break the game
+        used = script(f'transform odd:\n    {body}\nlabel start:\n    show eileen at odd\n    "Hi"')
         with pytest.raises(CompatibilityError, match="ATL"):
-            require_supported_scripts(data)
+            require_supported_scripts(used)
+        early = script(f'label start:\n    show eileen at odd\n    "Hi"\ntransform odd:\n    {body}\n')
+        with pytest.raises(CompatibilityError, match="ATL"):  # used before it is defined
+            require_supported_scripts(early)
 
 
 def test_renpy_atl_groups_events_and_named_transforms():