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/tests/test_compatibility.py

@@ -89,10 +89,37 @@ def test_unsupported_renpy_logic_never_degrades_to_ready(source, reason):
         require_supported_scripts(data)
 
 
-def test_renpy_atl_animation_is_rejected_instead_of_becoming_static():
-    data = script('transform moving:\n    linear 1.0 xalign 1.0\nlabel start:\n    "Hi"')
-    with pytest.raises(CompatibilityError, match="ATL"):
-        require_supported_scripts(data)
+def test_renpy_atl_animation_becomes_a_timeline_not_a_static_pose():
+    data = script('transform moving:\n    xalign 0.0\n    linear 1.0 xalign 1.0\n    repeat\nlabel start:\n    "Hi"')
+    require_supported_scripts(data)
+    assert data.transforms["moving"] == {"xalign": 0.0}
+    block = data.atl["moving"]
+    assert block["repeat"] is True
+    assert [step["op"] for step in block["steps"]] == ["set", "move"]
+    assert block["steps"][1]["props"] == {"xalign": 1.0} and block["steps"][1]["warp"] == "linear"
+
+
+def test_renpy_atl_constructs_without_a_runtime_are_still_rejected():
+    for body in ("function slide", "time 1.0:\n        xalign 1.0", "matrixcolor TintMatrix('#f00')",
+                 "contains:\n        'a.png'"):
+        data = script(f'transform odd:\n    {body}\nlabel start:\n    "Hi"')
+        with pytest.raises(CompatibilityError, match="ATL"):
+            require_supported_scripts(data)
+
+
+def test_renpy_atl_groups_events_and_named_transforms():
+    data = script(
+        "transform base:\n    zoom 0.8\n"
+        "transform bob:\n    base\n    parallel:\n        ease 1.0 yoffset -10\n        ease 1.0 yoffset 0\n"
+        "        repeat\n    parallel:\n        pause 0.5\n"
+        "    on hide:\n        linear 0.3 alpha 0.0\n"
+        'label start:\n    "Hi"')
+    require_supported_scripts(data)
+    steps = data.atl["bob"]["steps"]
+    assert steps[0] == {"op": "set", "props": {"zoom": 0.8}}  # known static transform is inlined
+    assert [s["op"] for s in steps] == ["set", "parallel", "on"]
+    assert len(steps[1]["blocks"]) == 2 and steps[1]["blocks"][0]["repeat"] is True
+    assert steps[2]["events"] == ["hide"]
 
 
 def test_source_encoding_is_preserved_or_explicitly_rejected(tmp_path):