feat: Ren'Py screens (screen/call screen/show screen) with actions and styles

Converter
- screens.py parses the screen language: containers (fixed, frame, vbox, hbox,
  grid, vpgrid, viewport, button, has), widgets (text, textbutton, imagebutton,
  add, bar ...), if/for/use/transclude, default, python, key, timer, on, inline ATL,
  conditional properties, and decompiled forms (unparenthesised operators,
  glued property values, comma separated actions)
- style statements, call/show/hide screen operators and Python-level
  renpy.show_screen are translated; only screens the game shows are audited,
  unused stock screens no longer block an import; undefined screens do
- ATL values may be computed at run time and transforms take parameters
  (at blink(0.2, 0.3)); blur property

Player
- screens.ts builds the DOM from the IR and rebuilds it on state changes;
  buttons with idle/hover/selected/insensitive states, tooltips, sounds, modal
  screens, timers and keys that survive re-rendering, saved with the save file
- Actions: Jump, Call, Return, SetVariable, Toggle*, SetScreenVariable, Show, Hide,
  ToggleScreen, Function, Play, Notify, If, Language, ShowMenu, OpenURL ...
- Crop/LiveCrop/Solid/Frame/Transform/DynamicDisplayable values, crop property,
  globals(), renpy.variant/change_language/seen_image, transition stubs

Tests: screen parser and runtime (linkedom DOM), screens fixture compiled from
tests/fixtures/screens.rpy. Docs: docs/SCREENS.md.

Twisted-Thoughts now passes the strict compatibility check.

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

backend/tests/test_compatibility.py

@@ -73,13 +73,15 @@ def test_renpy_only_declarations_are_not_execution_blockers(tmp_path):
     write(tmp_path / "game/script.rpy", source)
     assert inspect_directory(tmp_path)["compatible"]
     write(tmp_path / "game/script.rpy", source.replace('    "Hello"', '    call screen unused'))
-    with pytest.raises(CompatibilityError, match="call screen"):
+    assert inspect_directory(tmp_path)["compatible"]  # a defined screen can be called
+    write(tmp_path / "game/script.rpy", source.replace('    "Hello"', '    call screen missing_screen'))
+    with pytest.raises(CompatibilityError, match="screen"):
         inspect_directory(tmp_path)
 
 
 @pytest.mark.parametrize("source,reason", [
     ('init python:\n    def helper():\n        yield 1\nlabel start:\n    "Hi"', "init python"),
-    ('label start:\n    $ result = renpy.show_screen("map")', "renpy.show_screen"),
+    ('label start:\n    $ result = renpy.show_displayable("map")', "renpy.show_displayable"),
     ('label start:\n    jump missing', "missing"),
 ])
 def test_unsupported_renpy_logic_never_degrades_to_ready(source, reason):