feat(lib): add _assets module for bundled-weight resolution

Provide bundled_path() and resolve() helpers that locate ONNX
weights and JSON metadata via importlib.resources, falling back to
a user-supplied directory. Missing assets raise ModelLoadError.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/unit/test_assets.py

@@ -0,0 +1,34 @@
+"""Test the asset path resolver."""
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import pytest
+
+from ai_mouse import _assets
+from ai_mouse.errors import ModelLoadError
+
+
+def test_bundled_flow_model_exists() -> None:
+    p = _assets.bundled_path("flow_model.onnx")
+    assert p.exists()
+    assert p.suffix == ".onnx"
+
+
+def test_bundled_train_config_loadable() -> None:
+    p = _assets.bundled_path("train_config.json")
+    cfg = json.loads(p.read_text())
+    assert "seq_len" in cfg
+    assert "d_model" in cfg
+
+
+def test_resolve_with_custom_dir(tmp_path: Path) -> None:
+    (tmp_path / "flow_model.onnx").write_bytes(b"x")
+    p = _assets.resolve(tmp_path, "flow_model.onnx")
+    assert p == tmp_path / "flow_model.onnx"
+
+
+def test_missing_asset_raises_model_load_error(tmp_path: Path) -> None:
+    with pytest.raises(ModelLoadError, match="missing"):
+        _assets.resolve(tmp_path, "nonexistent.onnx")