Add pytest suite with 40 unit and integration tests

Coverage:
- test_model: SimpleNet forward (parametrized over batch sizes and both
  unsqueezed and flat input shapes), layer dimensions, differentiability,
  and ONNX round-trip
- test_inference: load_model resolution order (bundled, cwd override,
  explicit path, missing path), and predict shape/dtype/determinism plus
  endpoint sanity across 8 cardinal/diagonal targets
- test_train: _load_csv parsing, TrajectoryDataset indexing, full train()
  pipeline producing a single-file ONNX, plus a smoke test against the
  real data shipped under data/
- test_cli: --help for the three console scripts and a real run of
  mouse-visualize via both the entry point and python -m

Wire up pytest via dependency-groups and tool.pytest.ini_options.

pyproject.toml

@@ -30,6 +30,13 @@ mouse-collect = "mouse_control.collect:main"
 mouse-train = "mouse_control.train:main"
 mouse-visualize = "mouse_control.visualize:main"
 
+[dependency-groups]
+dev = ["pytest>=8.0"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+addopts = "-ra"
+
 [tool.hatch.build.targets.wheel]
 packages = ["src/mouse_control"]
 artifacts = ["src/mouse_control/assets/*.onnx"]