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/tests/test_screens.py

@@ -178,3 +178,19 @@ def test_mousearea_is_a_leaf_with_hover_actions():
     require_supported_scripts(data)
     node = data.screens["s"]["body"][0]
     assert node["w"] == "mousearea" and {"hovered", "unhovered", "xysize"} <= set(node["props"])
+
+
+def test_at_picks_a_transform_by_condition_and_atl_commas_split_statements():
+    data = parse('transform grow:\n    zoom 1.0\n    on selected:\n        zoom 1.2, 0.2\n    on deselected:\n'
+                 '        zoom 1.0, 0.2\ntransform still:\n    zoom 1.0\n'
+                 'screen s(flag):\n    frame at (grow if flag else still):\n        text "x"\n'
+                 'label start:\n    call screen s(True)\n')
+    audit_ir(data)
+    require_supported_scripts(data)
+    node = data.screens["s"]["body"][0]
+    choice = node["props"]["at"][0]["choose"]
+    assert choice["then"]["name"] == "grow" and choice["else"]["name"] == "still"
+    steps = data.atl["grow"]["steps"]
+    handler = steps[1]["block"]["steps"]
+    assert [s["op"] for s in handler] == ["set", "pause"] and handler[1]["t"] == 0.2
+    assert steps[1]["events"] == ["selected"] and steps[2]["events"] == ["deselected"]