feat: support user-defined functions and classes in Ren'Py python

- Converter: def/return/global/nonlocal, class (inheritance, property,
  staticmethod, classmethod), lambda, *args/**kwargs and call-site unpacking.
  Unsupported dunder methods, generators and unknown decorators are still refused
- Player: closures, super(), instances that survive save/load, isinstance,
  hasattr/getattr/setattr, map/filter/sorted(key=), recursion limit
- audit_ir accepts calls to names, classes and methods the game defines itself;
  unknown renpy.* calls still block the import. Volume/pan/pause/replay calls
  are accepted as presentation-only no-ops; build.* settings are skipped
- Tests: IR fixture compiled from tests/fixtures/classes.py (tests/make_ir.py)
  and run in the interpreter; converter tests updated

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

backend/tests/test_compatibility.py

@@ -78,7 +78,7 @@ def test_renpy_only_declarations_are_not_execution_blockers(tmp_path):
 
 
 @pytest.mark.parametrize("source,reason", [
-    ('init python:\n    def helper():\n        pass\nlabel start:\n    "Hi"', "init python"),
+    ('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    jump missing', "missing"),
 ])