fix: remove magenta error fallbacks and guard stale color strings in renderer
Replace magenta (255,0,255) error colors with black/transparent fallbacks
in findColor and plot. Add guards in box, lineh, and lineFast to reset
stale non-numeric color strings before drawing — prevents intermittent
magenta artifacts when mask + zebra ink state leaks across frames.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>