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>

frontend/src/player/engine.ts

@@ -112,6 +112,7 @@ export class Engine {
       resolveRef: (name) => this.resolveImage(name.split(" "))?.def ?? null,
       evalCond: (c) => truthy(this.safeEval(c)),
       transform: (name) => game.transforms[name] ?? null,
+      atl: (name) => game.atl?.[name] ?? null,
       text: (e) => this.interp(this.tr.str(pyStr(this.safeEval(e)))),
     });
     this.audio = new AudioSystem(this.settings, (f) => this.fileUrl(f), game.channels ?? {});