feat: conditional "at" transforms in screens and comma-separated ATL statements

`frame at (a if cond else b)` is kept as a choice that the player resolves each time the screen is
drawn. An ATL line splits at top-level commas into separate statements, so `zoom 1.2, 0.2` is a zoom
followed by a 0.2s pause, as Ren'Py compiles it (two-value properties such as `pos 0.5, 0.5` stay
whole). The inert events deselected/unselected, which Ren'Py never sends, are accepted. With the
build profile for The Only Doe 0.05 the game now passes the strict script check.

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

backend/app/converter/screens.py

@@ -401,6 +401,21 @@ class ScreenParser:
             spec["atl"] = animation
         props["at"] = [*props.get("at", []), spec]
 
+    def at_ref(self, source: str, line: Line) -> Node:
+        """One `at` entry; `(a if cond else b)` picks a transform while the screen is shown."""
+        text = source.strip()
+        if text.startswith("(") and text.endswith(")"):
+            try:
+                tree = ast.parse(text, mode="eval").body
+            except SyntaxError:
+                tree = None
+            if isinstance(tree, ast.IfExp):
+                cond = self.expr(ast.unparse(tree.test), line)
+                if cond is not None:
+                    return {"choose": {"c": cond, "then": self.at_ref(ast.unparse(tree.body), line),
+                                       "else": self.at_ref(ast.unparse(tree.orelse), line)}}
+        return self.transform_ref(source, line)
+
     def properties(self, words: list[str], line: Line, lenient: bool = False) -> dict[str, Any]:
         props: dict[str, Any] = {}
         i = 0
@@ -434,7 +449,7 @@ class ScreenParser:
                     i += 1
                     if not more:
                         break
-                props["at"] = [self.transform_ref(item, line) for item in items if item]
+                props["at"] = [self.at_ref(item, line) for item in items if item]
                 continue
             if glued is not None:
                 source, i = glued, i + 1