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

docs/patina/CHANGELOG.md

@@ -1,5 +1,22 @@
 # Patina changelog
 
+## Unreleased (0.5.0)
+
+- Each band now works like a small compressor. Add **Ratio** (1:1 to ∞:1,
+  default ∞:1): low ratios leave small bumps nearly alone and still cut big ones
+  hard. ∞:1 is the old behaviour.
+- Replace Selectivity (%) with **Threshold** (0–18 dB above the local floor,
+  default 3 dB, which equals the old default of 50 %).
+- Add a **Soft/Hard knee** switch. Soft widens the knee from 3 dB to 6 dB;
+  Hard uses a 1 dB knee and half the Attack time.
+- Show the gain curve (bump height in, cut out) under the knobs.
+- Mix and Output move to the slider panel under the plot.
+- Retune the Vocal harshness, Snare ring, Bright synth and Bass line presets
+  with Ratio and more Depth.
+- Breaking: FL Studio projects saved earlier load Threshold, Ratio and Knee at
+  their defaults. Settings files for `patina-render` with `selectivity` still
+  load and convert.
+
 ## 0.4.0
 
 - Add **Tone guard** (0–100 %, default 0 %). Patina follows the played note and

docs/patina/README.md

@@ -1,7 +1,7 @@
 # Patina
 
 Patina smooths a sound’s rough surface without erasing its character. Formerly
-**Resonance Compressor**; current version **0.4.0**.
+**Resonance Compressor**; current version **0.4.0** (0.5.0 unreleased).
 
 A mixing effect that finds harsh, ringing regions as they happen and pulls them
 down, while plain harmonic tone passes. Independently designed in Rust; no claim
@@ -20,18 +20,30 @@ redistributed packages.
 
 ## Work with it
 
-Three controls decide what happens:
+Each band works like a small compressor. It compares its level with the local floor
+around it and cuts only the part that sticks out:
 
-- **Depth** is the largest cut, in dB. 0 dB does nothing.
+- **Depth** is the largest cut per band, in dB. 0 dB does nothing.
 - **Sharpness** sets what counts as a resonance. Low values follow broad,
   smooth bumps (honk, boxiness). High values target narrow rings and whistles.
-- **Selectivity** sets how far a bump must stand out before it is cut. Low
-  values treat any bump; high values treat only the worst.
+- **Threshold** is how far (dB) a band must rise above its neighbours before it
+  is cut. Low values treat any bump; high values treat only the worst.
+- **Ratio** is how hard a bump above the threshold is cut. At n:1 a bump ends
+  1/n as far above the threshold as it started. Low ratios leave small bumps
+  nearly alone and still cut big ones; ∞:1 flattens every bump to the
+  threshold (this is how Patina worked before 0.5).
+- **Knee**: Soft fades cuts in around the threshold. Hard starts them right at
+  the threshold and uses half the Attack time.
 
-Start with Depth around 6 dB. Raise Selectivity until only the harsh parts show
-cuts, then use Delta to hear what is removed. Restrict Low/High frequency to keep
-the source’s body out of reach. Attack and Release set how quickly cuts grow and
-recover; Transient guard briefly relaxes cuts on attacks to keep punch.
+The small graph under the knobs shows the result: bump height above the floor
+in, cut out, with the threshold and the Depth limit.
+
+Start with Depth around 6 dB. Raise Threshold until only the harsh parts show
+cuts, then use Delta to hear what is removed. To hit big resonances hard and
+leave small ones alone, raise Depth to 12–20 dB and set Ratio to 2:1–4:1.
+Restrict Low/High frequency to keep the source’s body out of reach. Attack and
+Release set how quickly cuts grow and recover; Transient guard briefly relaxes
+cuts on attacks to keep punch.
 
 ### Tone guard for bass lines
 
@@ -61,7 +73,7 @@ on the plot drag the processing limits.
 On the plot, the grey fill is the measured input. The bright line is what the
 detector hears (its smoothed spectral envelope); the dim line is what it expects
 there. Accent-coloured cuts appear where the bright line rises above the dim one
-by more than Selectivity allows. The blue curve is the focus/protect shape.
+by more than Threshold allows. The blue curve is the focus/protect shape.
 
 Delta plays delayed dry minus wet before output trim. Use it to inspect changes,
 then return to normal monitoring to decide whether the music improved.
@@ -83,8 +95,9 @@ window size persist; Delta does not.
 4. The reference is a robust local straight-line fit over 2–3 octaves: spectral
    tilt is expected, bumps cannot raise their own reference, and empty regions
    (below a fundamental, beyond a filter cutoff) are ignored.
-5. Envelope minus reference, above the Selectivity threshold, becomes the cut,
-   limited by Depth, smoothed with Attack/Release, and linked across stereo.
+5. Envelope minus reference is the band's height above its floor. Above
+   Threshold it becomes a cut through the Knee and Ratio curve, limited by
+   Depth, smoothed with Attack/Release, and linked across stereo.
 
 ## Timing and limits
 
@@ -142,7 +155,9 @@ The CLAP ID is now `io.narl.bass-foundry.patina` (earlier builds used
 `io.storyboard.resonance-compressor`). Hosts treat Patina as a new plugin, so
 add it again in projects saved with an earlier build. Version 0.2 replaced
 Amount, Sensitivity, Max cut, Neighborhood, Cut width, and Knee with Depth,
-Sharpness, and Selectivity. Delete the old plugin file, place `Patina.clap`, and
+Sharpness, and Selectivity; version 0.5 replaced Selectivity with Threshold
+(dB), Ratio and Knee, so projects saved earlier load Threshold at its default.
+Delete the old plugin file, place `Patina.clap`, and
 rescan FL Studio.
 
 Source is grouped into `plugins/patina/{core,ui,render,plugin}`. See