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>

frontend/src/player/renderer.ts

@@ -91,7 +91,7 @@ export class Renderer {
       this.master.insertBefore(el, behind ?? later ?? null);
     }
     this.shown.set(tag, entry);
-    this.animate(entry, base, atSpecs);
+    this.animate(entry, base, [...(img.def.at ?? []), ...atSpecs]);
   }
 
   /** Start the ATL timelines of an image; each works on its own copy of the properties. */