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>

README.md

@@ -130,21 +130,22 @@ src/
     oversampler.rs  # ✅ 4x polyphase oversampler for true-peak detection (detection-only)
 ```
 
-The editor's meters and plot are currently drawn directly with egui's `Painter` inline in
-`editor.rs`. When the UI is redesigned (gain curve, draggable crossover, real layout), the plan is
-to split it into a widget module so each visualiser is self-contained and reusable:
+The editor lives in an `editor/` module — one file per visualiser widget (each owns its GUI
+state), with `mod.rs` as the aggregator/layout. Drawn directly with egui's `Painter`.
 
 ```
 src/
   editor/
-    mod.rs            # editor assembly + layout (replaces editor.rs)
-    widgets/
-      meter.rs        # |L | GR | R| level + gain-reduction cluster (extract from editor.rs)
-      plot.rs         # rolling in/out/GR scope (extract from editor.rs)
-      gain_curve.rs   # static gain-curve display per channel (threshold/ratio/knee)  — planned
-      crossover.rs    # frequency display with draggable crossover handles            — planned
+    mod.rs            # aggregator: create(), EditorState, layout, placeholder slider columns
+    meter.rs          # |L | GR | R| level + gain-reduction bars + per-channel ceiling lamp
+    plot.rs           # rolling in/out/GR scope (200 Hz ring feed) + ceiling-hit markers
+    crossover.rs      # log-freq strip with draggable crossover handles + number boxes
+    gain_curve.rs     # static gain-curve display (out vs in) for the selected channel
 ```
 
+Remaining UI work: replace the placeholder per-channel slider columns in `mod.rs` with the real
+layout.
+
 Deferred until the redesign — no need to split prematurely while the layout is still a placeholder.
  
 ---
@@ -195,12 +196,13 @@ is essential — without it FL silently skips a plugin it has seen before.)
  
 Work through these stages in order — each stage produces a loadable, audible plugin.
 
-**Status (2026-06-23):** Stages 1–4 done — the full signal chain works: 3-band LR4 crossover →
-per-band pre-gain + compressors (peak/RMS) → 'All' channel → **true-peak brickwall limiter** (4×
-oversampled detection). `lib.rs` has been split into `params.rs`, `editor.rs`, and `meters.rs`.
-Stage 6 metering is underway: per-channel **|L | GR | R| meters**, a **latching ceiling lamp**, and
-a **rolling in/out/gain-reduction plot** (per-channel tabs + flow-speed selector). **Next: gain-curve
-display and draggable crossover handles, then replace the placeholder slider UI.**
+**Status (2026-06-25):** Stages 1–4 done — the full signal chain works: 3-band LR4 crossover →
+per-band pre-gain + compressors (peak/RMS) → per-channel dry/wet mix → 'All' channel → **true-peak
+brickwall limiter** (4× oversampled detection). `lib.rs` is split into `params.rs`, `meters.rs`, and
+an `editor/` widget module. Stage 6 visualisers are essentially complete: per-channel **|L | GR | R|
+meters** + **per-channel ceiling lamps**, a **rolling in/out/GR plot** (200 Hz ring feed, flow-speed,
+ceiling-hit markers), **draggable crossover handles**, and a **static gain-curve display**. **Next:
+replace the placeholder slider columns with the real UI layout.**
  
 ### Stage 1 — Skeleton plugin ✅
 - [x] NIH-plug "passthrough" compiling and loading in DAW
@@ -236,8 +238,8 @@ display and draggable crossover handles, then replace the placeholder slider UI.
 - [x] Per-channel level meters (output level, `|L | GR | R|` cluster)
 - [x] Per-channel gain-reduction meters (vertical bars) + latching ceiling lamp
 - [x] Rolling in/out/gain-reduction plot (per-channel tabs, flow-speed selector)
-- [ ] Static gain-curve display per band (threshold/ratio/knee)
-- [ ] Draggable crossover handles on a frequency display
+- [x] Static gain-curve display (out vs in; includes pre-gain + makeup) for the selected channel
+- [x] Draggable crossover handles on a log-frequency display (with number boxes)
 - [ ] Replace the placeholder slider columns with the real UI
 ---