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/engine.ts

@@ -269,6 +269,8 @@ export class Engine {
       },
       stop: (...a: any[]) => this.audio.stop(kwOf(a).channel ?? ch, kwOf(a).fadeout ?? 0),
       set_volume: () => {},
+      set_pause: () => {},
+      set_pan: () => {},
       get_playing: () => this.audio.current(ch)?.file ?? null,
       is_playing: () => this.audio.current(ch) !== null,
     });
@@ -301,6 +303,10 @@ export class Engine {
         throw new JumpSignal("jump", "__quit__");
       },
       version: () => "pgvn",
+      // presentation-only: nothing to roll back or replay, and the next transition is chosen by `with`
+      end_replay: () => null,
+      block_rollback: () => null,
+      transition: () => null,
     };
     const renpy = new Proxy(renpyImpl, {
       get: (t, k: string) => {