feat: add damp_start (smoothstep lateral damping, no release kink)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/unit/test_postprocess.py

@@ -182,3 +182,27 @@ def test_soften_forward_no_lower_clip() -> None:
     f = np.array([0.0, -0.01, 0.30, 0.70, 1.0])
     out = soften_forward(f)
     assert out[1] < 0.0
+
+
+from ai_mouse._postprocess import damp_start
+
+
+def test_damp_start_dampens_early_lateral() -> None:
+    lat = np.full(16, 1.0)
+    out = damp_start(lat, n=4)
+    assert out[1] < out[2] < out[3] < out[4] < 1.0  # monotone ramp
+    assert np.all(out[5:] == 1.0)  # untouched past n
+
+
+def test_damp_start_no_release_jump() -> None:
+    # The weight at i=n must be close to 1 (continuous release):
+    # smoothstep(4/5) = 0.896, vs the old linear 4/5 = 0.8.
+    lat = np.full(16, 1.0)
+    out = damp_start(lat, n=4)
+    assert out[4] > 0.85
+
+
+def test_damp_start_short_input_safe() -> None:
+    lat = np.array([0.0, 0.5, 0.3])
+    out = damp_start(lat, n=4)  # n capped to len//4 = 0 → no-op
+    assert np.array_equal(out, lat)