Diversify palette by timbre, tame the grain

Palette hue was pinned to the spectral centroid, which is essentially a
bass-vs-treble number most mastered pop lands mid-range on, so different
songs converged on the same blue/green/purple wedge and warm red/yellow
was unreachable. Derive the base hue from a timbre signature instead:
the track's spectral mass in the body (sub/low/mid) against the trebles
(high/air), mapped onto a cool(blue)->warm(red) ramp, with BPM+tonality
+dynamics driving vibrance. Energy and timbre are now two orthogonal
axes, and palettes vary meaningfully between tracks.

Grain was also overused: every scene adds its own surface grain and the
grade adds another on top. Drop the per-scene texture floor, the grade's
grain range, and the default, so tonal tracks read clean.

flow-state/src/engine/Compositor.js

@@ -10,7 +10,7 @@ const DEFAULT_POST = {
     bloomThreshold: 0.6,
     bloomKnee: 0.3,
     chroma: 0.15,
-    grain: 0.04,
+    grain: 0.02,
     vignette: 0.35,
     contrast: 1.05,
     saturation: 1.1,