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/tests/fixtures/exceptions.py

@@ -0,0 +1,99 @@
+# Source of exceptions.ir.json. Regenerate from backend/ with:
+#   python -m tests.make_ir ../frontend/tests/fixtures/exceptions.py ../frontend/tests/fixtures/exceptions.ir.json
+
+
+class AppError(Exception):
+    def __init__(self, msg, code=0):
+        super().__init__(msg)
+        self.code = code
+
+
+def risky(n):
+    if n == 0:
+        raise AppError("zero", code=7)
+    return 10 // n
+
+
+log = []
+try:
+    risky(0)
+except AppError as e:
+    log.append(str(e))
+    result_code = e.code
+else:
+    log.append("no")
+finally:
+    log.append("fin")
+
+try:
+    d = {"a": 1}
+    v = d["b"]
+except KeyError:
+    result_key = "keyerror"
+
+try:
+    xs = [1]
+    xs[5]
+except (IndexError, ValueError):
+    result_index = "indexerror"
+
+try:
+    int("abc")
+except ValueError:
+    result_value = "valueerror"
+
+try:
+    risky(0)
+except Exception:
+    result_generic = True
+
+try:
+    result_zero = 1 / 0
+except ZeroDivisionError:
+    result_zero = "zerodiv"
+
+
+def retry():
+    for i in range(3):
+        try:
+            if i < 2:
+                raise ValueError("again")
+            return i
+        except ValueError:
+            continue
+    return -1
+
+
+result_retry = retry()
+
+found = None
+for n in [1, 3, 5]:
+    if n % 2 == 0:
+        found = n
+        break
+else:
+    found = "none"
+result_forelse = found
+
+k = 0
+while k < 3:
+    k += 1
+else:
+    k = k * 10
+result_whileelse = k
+
+try:
+    try:
+        raise RuntimeError("inner")
+    except RuntimeError:
+        raise
+except RuntimeError as e3:
+    result_reraise = str(e3)
+
+result_pairs = [(x, y) for x in [1, 2] for y in [3, 4]]
+result_dictcomp = {a: b for a in [1, 2] for b in [a * 10] if b > 10}
+
+try:
+    assert 1 == 2, "boom"
+except AssertionError as e4:
+    result_assert = str(e4)