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/compatibility.py

@@ -103,7 +103,7 @@ CALLS |= {f"renpy.{channel}.{method}" for channel in ("music", "sound")
 # rollback bookkeeping). They never change the story state.
 CALLS |= {f"renpy.{channel}.{method}" for channel in ("music", "sound")
           for method in ("set_volume", "set_pause", "set_pan")}
-CALLS |= {"renpy.end_replay", "renpy.block_rollback"}
+CALLS |= {"renpy.end_replay", "renpy.block_rollback", "renpy.transition"}
 CALLS |= {"map", "filter", "hasattr", "getattr", "setattr", "callable", "super"}
 METHODS = set("append extend insert pop remove index count reverse sort clear copy add discard get keys values items setdefault update lower upper capitalize title strip lstrip rstrip startswith endswith replace split join format find isdigit isalpha".split())