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.

tests/conftest.py

@@ -0,0 +1,32 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+DATA_DIR = REPO_ROOT / "data"
+
+
+@pytest.fixture(scope="session")
+def train_csv() -> Path:
+    path = DATA_DIR / "mouse_data.csv"
+    if not path.is_file():
+        pytest.skip(f"missing training csv: {path}")
+    return path
+
+
+@pytest.fixture(scope="session")
+def test_csv() -> Path:
+    path = DATA_DIR / "mouse_data_test.csv"
+    if not path.is_file():
+        pytest.skip(f"missing test csv: {path}")
+    return path
+
+
+@pytest.fixture(scope="session")
+def bundled_session():
+    """Shared inference session for the bundled model (loading is slow)."""
+    from mouse_control import load_model
+
+    return load_model()