feat(lib): add errors module

Introduce AiMouseError base class plus ModelLoadError and
GenerationError subclasses so downstream consumers can catch the
umbrella or specific failure modes.

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

src/ai_mouse/errors.py

@@ -0,0 +1,18 @@
+"""Exception hierarchy for the ai_mouse library.
+
+Downstream consumers can catch the umbrella :class:`AiMouseError`
+or the specific subclasses for finer control.
+"""
+from __future__ import annotations
+
+
+class AiMouseError(Exception):
+    """Base class for all ai_mouse errors."""
+
+
+class ModelLoadError(AiMouseError):
+    """Raised when ONNX weights / metadata cannot be loaded."""
+
+
+class GenerationError(AiMouseError):
+    """Raised when inference produces an invalid result (e.g. NaN)."""

tests/unit/test_errors.py

@@ -0,0 +1,19 @@
+"""Test the error hierarchy."""
+from __future__ import annotations
+
+import pytest
+
+from ai_mouse import errors
+
+
+def test_model_load_error_is_aimouse_error() -> None:
+    assert issubclass(errors.ModelLoadError, errors.AiMouseError)
+
+
+def test_generation_error_is_aimouse_error() -> None:
+    assert issubclass(errors.GenerationError, errors.AiMouseError)
+
+
+def test_can_catch_specific_with_general() -> None:
+    with pytest.raises(errors.AiMouseError):
+        raise errors.ModelLoadError("test")