feat: add ATL animation timelines and try/raise to the Ren'Py player

Python sandbox
- try/except/else/finally, raise (incl. re-raise), assert, for/while ... else
- Multi-generator and dict comprehensions
- Builtin exception hierarchy; user exceptions subclass it
- Inside try, missing keys/indexes, division by zero, attribute access on None
  and unknown names raise Python exceptions; lookups stay lenient elsewhere
- int()/float() raise ValueError on non-numeric strings

ATL
- converter/atl.py: set/move (linear, ease, easein, easeout), pause, repeat,
  parallel, choice, block, on show/hide and named transforms become a timeline
  shipped as game.json "atl"; Move() becomes a linear slide
- player/atl.ts: timeline runner wired into the renderer; `on hide` plays
  before the picture is removed
- function, time, contains, matrixcolor and expressions still block the import

Tests: exceptions/ATL fixtures and runtime tests, converter ATL tests; README
and ARCHITECTURE updated.

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

backend/app/converter/compatibility.py

@@ -141,6 +141,10 @@ def _bound_names(root) -> tuple[set[str], set[str]]:
                 names.update(p["n"] for p in value["params"])
             elif kind == "comp":
                 target(value.get("t"))
+                for gen in value.get("gens", []):
+                    target(gen.get("t"))
+            elif op == "try":
+                names.update(h["name"] for h in value.get("handlers", []) if h.get("name"))
             for key, child in value.items():
                 visit(child, op == "class" and key == "body")