refactor(lib): remove legacy generator.py / coord.py / scroll module

Drop the pre-migration PyTorch inference pipeline now that the ONNX-backed
MouseModel/ScrollModel in mouse.py and scroll.py are wired up through the
public ai_mouse API.

Deleted:
  * src/ai_mouse/generator.py        (legacy torch flow ODE + post-processing)
  * src/ai_mouse/coord.py            (legacy public coord transforms,
                                      superseded by ai_mouse._coord)
  * src/ai_mouse/_scroll_legacy.py   (legacy torch scroll VAE inference)
  * scripts/build_golden_*.py        (one-shot capture scripts, no longer
                                      needed once goldens are committed)
  * tests/unit/test_generator.py     (legacy module gone)
  * tests/unit/test_scroll_generator.py (legacy module gone)
  * tests/unit/test_coord.py         (legacy module gone; ai_mouse._coord is
                                      tested by test__coord.py)
  * scripts/                         (empty, removed)

Tools migrations:
  * tools/trainer.py: import encode_trajectory from ai_mouse._coord
    instead of the deleted ai_mouse.coord
  * tools/server/routes_verify.py, tools/server/routes_scroll.py: route to
    the public ai_mouse.generate / generate_scroll. They no longer accept
    a model_dir override — the bundled ONNX is the source of truth, and a
    fresh export goes through `python -m tools.export_onnx`.
  * tools/eval/__main__.py: same migration; model_dir CLI arg retained as
    a deprecation shim but ignored.

Final src/ai_mouse/ layout (matches plan):
  __init__.py, _assets.py, _coord.py, _model_cache.py, _postprocess.py,
  errors.py, mouse.py, py.typed, scroll.py, assets/

Test suite: 188 passed (was 188 before deletion; obsolete suites cleaned
out alongside the modules they covered).

tools/server/routes_scroll.py

@@ -182,21 +182,17 @@ async def scroll_train(req: ScrollTrainRequest) -> StreamingResponse:
 
 @router.post("/verify")
 def scroll_verify(req: ScrollVerifyRequest) -> dict:
-    from ai_mouse.scroll.generator import generate_scroll
+    # Uses the bundled ONNX scroll model exposed via the public ai_mouse API.
+    # The legacy scroll_model.pt path is no longer wired in; export a fresh
+    # scroll_decoder.onnx via `python -m tools.export_onnx` to update.
+    from ai_mouse import generate_scroll
 
-    _, models_dir = _paths()
-    if not (models_dir / "scroll_model.pt").exists():
-        raise HTTPException(
-            status_code=400,
-            detail="滚轮模型尚未训练，请先在「训练模型 → 滚轮模型」中完成训练。",
-        )
     paths = []
     for _ in range(min(req.n_paths, 12)):
         events = generate_scroll(
             req.start_scrollY,
             req.target_scrollY,
             mode=req.mode,
-            model_dir=str(models_dir),
         )
         paths.append(events)
     return {"paths": paths}