Also: calibrate._sample_rgb now snaps to the most-saturated pixel within 15px
of the click, so rough clicks still pick up the dot's pure colour. Default
dot-colour tolerance bumped 30→60 to absorb anti-aliasing.
Test fixture _SAMPLED_RGB recomputed for the new 36/49 dilution (was 24/49
when sampling at the trailing edge).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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