colour: the ±12 decoy controls read dead zeros and inflate the excess statistic

Found during PR #162 review (pre-existing, orthogonal to the width change): the ±12 deg colour decoys read a hard 0.00% on every lag because reach defaults to 8 and they lie entirely outside the profile window — and excess = seam − mean(all four decoys) averages the two dead zeros in. On the review's test view this inflates the 32px excess from 4.47% to 5.61%.

Fix directions: clamp decoys inside the profile window, or exclude out-of-window decoys from the mean, or widen reach with the decoys. Filed under the owner's standing issue authorization.