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/app/converter/pipeline.py

@@ -292,9 +292,14 @@ def _size(path: Path) -> int:
     return sum(p.stat().st_size for p in path.rglob("*") if p.is_file())
 
 
+# Directories inside game/ that are not the story: engine caches, saves, translations, and
+# add-on tooling players drop in (0x52-URM is an in-game console / save editor).
+ENGINE_DIRS = ("cache", "saves", "tl", "renpy", "0x52-urm")
+
+
 def _is_engine_file(game_dir: Path, p: Path) -> bool:
     parts = [x.lower() for x in p.relative_to(game_dir).parts]
-    return len(parts) > 1 and parts[0] in ("cache", "saves", "tl", "renpy")
+    return len(parts) > 1 and parts[0] in ENGINE_DIRS
 
 
 def _resolve(path: str, aliases: dict[str, str]) -> str | None: