Replace per-frame hsl(val*120) with a static createLinearGradient
(green->yellow->red, bottom to top) shared across all bars. Colour is
now position-based, not amplitude-based, so it never flickers. Peak
decay slightly slower (0.008/frame) for smoother hold.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>