Add unit tests for SSM helper functions, replacing always-failing stub

[Copilot]

tests/test_ssm.py was a placeholder containing test_always_passes and test_always_fails, breaking the test suite with zero coverage of the SSM module.

Changes

• tests/test_ssm.py — replaced stub with 19 unit tests covering the two pure-NumPy helpers in musicalgestures/_ssm.py (no FFmpeg or real video required):

TestSmoothDownsampleFeatureSequence

• Output shape under default params, custom downsampling, and single-feature input
• Sampling rate reduction (sr / down_sampling)
• Smoothing correctness: constant signal survives unchanged (interior samples), noisy signal variance is reduced
• Returned FuncFormatter is callable and produces correct string output
• All supported window types (boxcar, hann, hamming, blackman) produce valid output
• Output dtype is always floating-point

TestSlowDot

• Result matches np.dot for random matrices
• Identity, shape, single-row, and zero-matrix cases
• X @ Xᵀ is symmetric
• Diagonal dominance holds for cosine-similarity SSMs built from normalised rows

# Example: SSM symmetry property now tested
X = rng.random((15, 8))
S = slow_dot(X, X.T, length=15)
np.testing.assert_allclose(S, S.T, atol=1e-12)