WGSL: add octaveBrightness() to v0.46, v0.47, v0.48 for note-color parity

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Context

Follow-up from shader playback audit (PR #268). WebGL overlay now applies octaveBrightness() so the same pitch class shares hue across octaves but gets brighter in higher octaves (0.65 at C-0 → 1.0 at B-9), matching patternv0.50.wgsl.

Older LED shaders still use pitch-class color only — C-3 and C-5 look identical.

Scope

Add octaveBrightness(note) and apply it wherever note color is computed in:

• shaders/patternv0.46.wgsl
• shaders/patternv0.47.wgsl
• shaders/patternv0.48.wgsl

Reference implementation:

fn octaveBrightness(note: u32) -> f32 {
  if (note == 0u || note > NOTE_MAX) { return 1.0; }
  let oct = (note - 1u) / 12u; // 0..9
  return 0.65 + 0.35 * f32(oct) / 9.0;
}

Then run npm run sync:shaders to copy into public/shaders/.

Acceptance criteria

• Same pitch class (e.g. all C notes) shares hue across octaves
• Higher octaves are visibly brighter than lower octaves
• Sustain tail rows inherit octave-scaled pitch color (dimmed, not re-hued)
• No change to GPU packing or uniform layout

Related

• docs/planning/trigger-sustain-tails.md
• PR Fix WebGL overlay placement and octave note-color parity #268

[linear-code]

JUL-799