feat(lib): add private _coord.py with numpy transforms

Copy of coord.py (which is already pure numpy) into the private
underscored module to be consumed by upcoming mouse.py rewrite.

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

tests/unit/test__coord.py

@@ -0,0 +1,30 @@
+"""Test the private numpy coordinate transforms."""
+from __future__ import annotations
+
+import numpy as np
+
+from ai_mouse._coord import decode_trajectory, encode_trajectory
+
+
+def test_encode_decode_roundtrip() -> None:
+    points = np.array([[100.0, 200.0], [300.0, 250.0], [500.0, 300.0]])
+    start = (100, 200)
+    end = (500, 300)
+    encoded = encode_trajectory(points, start, end)
+    decoded = decode_trajectory(encoded, start, end)
+    assert np.allclose(decoded, points, atol=1e-6)
+
+
+def test_encode_endpoints() -> None:
+    """Start should encode to (0,0); end should encode to (1,0)."""
+    points = np.array([[100.0, 200.0], [500.0, 300.0]])
+    encoded = encode_trajectory(points, (100, 200), (500, 300))
+    assert np.allclose(encoded[0], [0.0, 0.0], atol=1e-6)
+    assert np.allclose(encoded[1], [1.0, 0.0], atol=1e-6)
+
+
+def test_zero_distance_returns_zeros() -> None:
+    points = np.array([[100.0, 200.0]])
+    encoded = encode_trajectory(points, (100, 200), (100, 200))
+    assert encoded.shape == (1, 2)
+    assert np.all(encoded == 0)