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/make_ir.py

@@ -0,0 +1,17 @@
+"""Compile a Python file to PGVN IR JSON: python -m tests.make_ir SOURCE.py OUT.json"""
+import json
+import sys
+from pathlib import Path
+
+from app import pgvn
+from app.converter import pyexpr
+
+
+def main(source: Path, out: Path) -> None:
+    code = pyexpr.parse_stmts(source.read_text(encoding="utf-8"))
+    pgvn.statements(code)
+    out.write_text(json.dumps(code, ensure_ascii=False, separators=(",", ":")), encoding="utf-8")
+
+
+if __name__ == "__main__":
+    main(Path(sys.argv[1]), Path(sys.argv[2]))