feat(patina): Tone guard for bass lines

Patina detects the played note (30-500 Hz) and remembers, over about
4 s, how high each spectral bump is at a fixed frequency and relative
to the note. Bumps that follow the note (a boosted harmonic the patch
is built on) get less cut; bumps fixed in Hz (formants, room modes)
are cut as before. On a saw line F1-F2 with harmonic 6 boosted, the
cut on that harmonic drops from 5.78 dB to 0.44 dB at 100 %, while a
fixed formant at 1.5 kHz stays at 5.78 dB. One held note is cut as
normal, since the two cases cannot be told apart.

- New parameter tone-guard (0-100 %, default 0), appended at the end;
  0 % is the old behaviour and old settings load unchanged.
- New Bass line preset.
- Editor: Tone guard control, NOTE readout and a strip over the
  spectrum where cuts are held back.
- No added latency, no allocation on the audio thread; patina-core now
  depends on common-dsp.

docs/patina/CHANGELOG.md

@@ -1,5 +1,15 @@
 # Patina changelog
 
+## Unreleased
+
+- Add **Tone guard** (0–100 %, default 0 %). Patina follows the played note and
+  remembers, over about four seconds, which bumps stay in Hz and which follow the
+  note. Bumps that follow the note (a harmonic the bass is built on) are cut
+  less; resonances fixed in Hz (formant, comb and filter rings) are still cut.
+  At 0 % Patina works exactly as before. The display shows the note and marks
+  the held-back region with a blue strip.
+- Add the **Bass line** preset (Tone guard 80 %, Low 150 Hz).
+
 ## 0.3.0
 
 - Change the CLAP ID to `io.narl.bass-foundry.patina`. Hosts see this as a new

docs/patina/README.md

@@ -33,6 +33,27 @@ cuts, then use Delta to hear what is removed. Restrict Low/High frequency to kee
 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
+
+On a bass line, a bump can mean two things. A resonance fixed in Hz (a formant,
+a comb or filter ring) stays at the same frequency on every note: that is what
+Patina should cut. A bump that follows the note (a harmonic the patch is built
+on, or a Lathe boost) moves with every note: cutting it changes the patch
+differently on each note.
+
+**Tone guard** tells them apart. Patina detects the played note (30–500 Hz)
+and remembers, over about four seconds, how high each bump was in Hz and how
+high it was relative to the note. A bump that has stayed with the note more
+than in Hz gets less cut; at 100 % none. A resonance fixed in Hz is cut as
+before. It needs a few different notes before it can decide: on one held note
+both look the same and Patina cuts as normal. The **NOTE** readout shows what
+Patina follows, and a blue strip on top of the plot shows where cuts are held
+back. Start from the **Bass line** preset. At 0 % Tone guard is off.
+
+Measured on a saw line F1–F2 with Low at 150 Hz: a +12 dB boost on harmonic 6
+is cut by 5.8 dB without Tone guard and by 0.4 dB at 100 %; a formant at 1.5 kHz
+is cut by 5.8 dB either way.
+
 Double-click the plot to add one of four focus points. Drag a point up to cut
 more in that region, down to protect it. Wheel changes its width. Range handles
 on the plot drag the processing limits.
@@ -76,7 +97,8 @@ The engine uses complex RustFFT with supplied scratch buffers, WOLA reconstructi
 the envelope/reference detector above, bounded soft-knee cuts, and temporal
 smoothing. Original bin phases are retained. This is not a minimum-phase filter
 bank, a live monitoring effect, a limiter, or an automatic separation of tone and
-unwanted resonance. No external sidechain, Learn, M/S, or pitch detector in v0.1.
+unwanted resonance. No external sidechain, Learn or M/S. The pitch detector
+serves Tone guard only.
 
 ## Build and offline evaluation