feat(patina): per-band compressor gain stage with Threshold, Ratio and Knee

Tester feedback asked for a threshold and a ratio. Each band now cuts like a
small compressor: Threshold (dB above the local floor) replaces Selectivity,
Ratio scales the cut per dB above it (inf:1 is the old behaviour), and a
Soft/Hard knee switch sets the curve. A small graph shows bump height in, cut
out. Mix and Output move to the slider panel so the detection panel fits.
common-ui gains rotary_text for knobs with custom value text.

docs/ideas.md

@@ -237,7 +237,19 @@ one period of the note, so heavy settings do not ripple at f0. See [Planish](pla
 - **Knurl**: velocity or note selects the pattern. Stereo grit above a split
   only. Drift (planned for 0.3).
 - **Patina**: Tone guard (done, Patina 0.4.0). MIDI as its pitch source. M/S
-  processing.
+  processing. Threshold, Ratio and Soft/Hard knee from tester feedback (done,
+  unreleased 0.5.0). Next, from the same feedback round:
+  - An absolute level gate (dBFS) so quiet passages are never cut, if the
+    tester meant a loudness threshold rather than a prominence threshold.
+  - Narrow rings on dense or noisy material are under-read: a +12 dB ring
+    (Q 25) on noise reads only 3–6 dB above the floor, so it is cut by 1–3 dB.
+    Two causes, measured: the joined-tops envelope lifts the floor to the noise
+    peaks (about 5 dB above the noise mean), and envelope smoothing loses about
+    3 dB on a narrow ring. A floor that follows Sharpness (1/3–2 octaves) was
+    tried and rejected: it lost a resonant lowpass, absorbed broad bumps and cut
+    plain harmonic tones. Better candidates: join tops only across deep gaps
+    (harmonic spacing), or a mean-based floor for noise-like regions.
+  - A finer analysis grid (1024 cells) for narrow rings at high frequencies.
 - **Weld**: delay spread display (done, Weld 0.3.0). A *gentle* fit option
   that keeps the delay spread under a limit, for plucky layers. Alignment slots
   per note or per switch-up. The External