feat: static gain-curve display for the selected channel

Add editor/gain_curve.rs: a square panel beside the scrolling plot showing the
channel transfer (out vs in, -60..0 dB) for whichever channel the plot tab
selects. Plots the full wet path — out = (in + pre_gain) + gain_reduction + makeup
— using the shared Compressor::gain_computer (now pub) so it matches the DSP and
the GR meter. Unity-reference diagonal + threshold marker; mix not folded in.

Update README structure/status to reflect the editor/ widget module and the
completed Stage 6 visualisers.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/editor/mod.rs

@@ -20,6 +20,7 @@ use crate::params::{Codename206Params, CompressorParams};
 use crate::Codename206;
 
 mod crossover;
+mod gain_curve;
 mod meter;
 mod plot;
 
@@ -75,7 +76,12 @@ pub(crate) fn create(params: Arc<Codename206Params>, meters: Arc<Meters>) -> Opt
                         ui.heading(Codename206::NAME);
                         meter::draw(ui, &meters, &mut state.meter);
                         ui.separator();
-                        plot::draw(ui, &meters, &mut state.plot);
+                        // Gain curve (left, square) beside the scrolling plot (right, fills the rest).
+                        let selected = state.plot.selected;
+                        ui.horizontal_top(|ui| {
+                            ui.vertical(|ui| gain_curve::draw(ui, &params, selected));
+                            ui.vertical(|ui| plot::draw(ui, &meters, &mut state.plot));
+                        });
                         ui.separator();
                         crossover::draw(ui, &params, setter);
                         ui.separator();