feat(patina): rebuild resonance detector, simplify controls, new look

Patina 0.2.0. The old detector compared raw FFT bins with a one-octave
dB mean, so every harmonic of a tonal sound looked like a resonance and
real harsh bumps were barely touched. The new detector:

- averages power over each log band and smooths it over time
- joins harmonic tops into an envelope, smoothed by Sharpness
- compares it with a robust, tilt-aware local line fit that ignores
  empty regions and cannot be lifted by the bump itself

Six overlapping controls become Depth, Sharpness and Selectivity; focus
points now read "up cuts more, down protects". The analysis window is
halved to 2048 samples (about 43 ms delay). Behaviour tests cover plain
harmonic tones, resonant bumps, narrow rings, level independence,
flicker on noise and high sample rates.

Both editors get a dark metallic look with #FF0160 as the single
accent colour; screenshots and docs are updated. Projects saved with
Patina 0.1 load the new controls at their defaults.

docs/rivet/CHANGELOG.md

@@ -1,5 +1,9 @@
 # Rivet changelog
 
+## Unreleased
+
+- New dark metallic look with a single accent colour, shared with Patina.
+
 ## 0.3.0
 
 - Introduce the Rivet name and a dedicated product source/documentation tree.