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/test_cli.py

@@ -0,0 +1,49 @@
+from __future__ import annotations
+
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+CLI_COMMANDS = ["mouse-collect", "mouse-train", "mouse-visualize"]
+
+
+@pytest.mark.parametrize("command", CLI_COMMANDS)
+def test_cli_help(command: str) -> None:
+    """Every CLI must respond to --help with exit 0 and an argparse usage line."""
+    result = subprocess.run(
+        [command, "--help"], capture_output=True, text=True, check=False,
+    )
+    assert result.returncode == 0, result.stderr
+    assert "usage:" in result.stdout
+
+
+def test_visualize_no_show_produces_png(tmp_path: Path) -> None:
+    save_path = tmp_path / "out.png"
+    result = subprocess.run(
+        [
+            "mouse-visualize", "--no-show", "--seed", "1",
+            "--n-trajectories", "3", "--n-interp", "20",
+            "--save", str(save_path),
+        ],
+        capture_output=True, text=True, check=False,
+    )
+    assert result.returncode == 0, result.stderr
+    assert save_path.exists()
+    assert save_path.stat().st_size > 1000
+
+
+def test_visualize_module_invocation(tmp_path: Path) -> None:
+    """The module is also runnable via `python -m mouse_control.visualize`."""
+    save_path = tmp_path / "out.png"
+    result = subprocess.run(
+        [
+            sys.executable, "-m", "mouse_control.visualize",
+            "--no-show", "--seed", "2", "--n-trajectories", "2",
+            "--save", str(save_path),
+        ],
+        capture_output=True, text=True, check=False,
+    )
+    assert result.returncode == 0, result.stderr
+    assert save_path.exists()