Converter: cover real Ren'Py games better, add folder import and dry run

Checked against six real games (Ren'Py 7.5 and 8.x):
- nested labels and named menus become jump anchors inside label bodies
- call with arguments and labels with parameters (dynamically scoped)
- frame-by-frame ATL images, Movie frames, layeredimage, camera, Shake,
  custom audio channels via register_channel, show text with expressions
- generator expressions in Python, no SyntaxWarning noise from decompiled code

Also:
- app.import_dir converts an unpacked game folder without changing it
- app.converter.analyze dry-runs the script part of the converter
- imports use their own "importing" status so a worker restart can't hijack them

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

backend/tests/test_converter.py

@@ -79,3 +79,44 @@ def test_full_pipeline(data_dir, tmp_path):
     assert [lbl["name"] for lbl in script["labels"]] == ["start", "walk", "stay_home", "ending"]
     if shutil.which("ffmpeg"):
         assert "audio/theme.ogg" in files
+
+
+def test_nested_labels_and_named_menus_become_anchors():
+    from app.converter.pipeline import anchors
+    d = parse("label start:\n    if x:\n        label inner:\n            \"a\"\n"
+              "    menu pick:\n        \"A\":\n            jump .loc\n"
+              "label .loc:\n    return\n")
+    labels = d.modules["t.rpy"]
+    assert [lbl["name"] for lbl in labels] == ["start", "inner.loc"]  # local labels follow the last global one
+    found = anchors(labels)
+    assert found["inner"] == {"label": "start", "path": [0, 0, 0], "params": None}
+    assert found["pick"]["path"] == [1]
+    assert labels[0]["body"][2]["items"][0]["body"][0]["target"] == "inner.loc"
+
+
+def test_call_arguments_and_label_params():
+    d = parse('label start:\n    call greet("Bob", loud=True) from _c1\n'
+              'label greet(name, loud=False):\n    "Hi [name]"\n    return\n')
+    call = d.modules["t.rpy"][0]["body"][0]
+    assert call["target"] == "greet" and call["args"]["a"][0]["v"] == "Bob"
+    assert "loud" in call["args"]["kw"]
+    params = d.modules["t.rpy"][1]["params"]
+    assert params[0] == ["name", None] and params[1][1]["v"] is False
+
+
+def test_atl_frames_layeredimage_and_channels():
+    d = parse('image blink:\n    "a.png" with dissolve\n    pause 0.5\n    "b.png"\n    0.25\n'
+              '    repeat\n'
+              'image vid:\n    Movie(play="v.webm")\n'
+              'layeredimage eileen:\n    always:\n        "eileen_base"\n'
+              '    group face:\n        attribute happy default\n        attribute sad "sad.png"\n'
+              'init python:\n    renpy.music.register_channel("amb", mixer="sfx", loop=True)\n')
+    blink = d.images["blink"]
+    assert [f.get("d") for f in blink["frames"]] == [0.5, 0.25] and blink["repeat"]
+    assert d.images["vid"] == {"movie": "v.webm"}
+    layers = d.images["eileen"]["layered"]
+    assert layers[0] == {"always": {"ref": "eileen_base"}}
+    assert layers[1] == {"attr": "happy", "group": "face", "default": True,
+                         "img": {"ref": "eileen_face_happy"}}
+    assert layers[2]["img"] == {"file": "sad.png"}
+    assert d.channels["amb"] == {"mixer": "sfx", "loop": True}