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                                      superseded by ai_mouse._coord)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/trainer.py

@@ -23,7 +23,7 @@ import numpy as np
 import torch
 from torch.utils.data import DataLoader
 
-from ai_mouse.coord import encode_trajectory
+from ai_mouse._coord import encode_trajectory
 from tools.config import TrainConfig
 from tools.models import TrajectoryFlowModel
 from tools.utils import resample_arc