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/plot.rs

@@ -62,8 +62,8 @@ impl PlotHistory {
 /// GUI-side state for the plot: selected channel, history ring, ring-drain cursor, and the
 /// column being assembled from drained buckets.
 pub(super) struct PlotState {
-    /// Channel shown in the plot (0..NUM_CHANNELS: low/mid/high/all).
-    selected: usize,
+    /// Channel shown in the plot (0..NUM_CHANNELS: low/mid/high/all). Also drives the gain curve.
+    pub(super) selected: usize,
     history: PlotHistory,
     /// Seconds of history shown across the full plot width — the flow speed (smaller = faster).
     window_s: f64,