Show a Gain node as an orange device with one knob (#384)

A Gain node now draws a small device in the graph, as the Compressor
does since #380: the brushed plate in orange, 160x220 graph units, with
one chrome knob between its inputs and its output.

- GainPanel and GainFace join the node faces. The knob drags, resets
  on a double-click and keeps the wheel for the graph (#383). Each turn
  is one undo step, a linked Gain (dB) greys it.
- The knob turns with a taper of 0.26, so 0 dB stands straight up and
  the range below -24 dB takes a fifth of the turn.
- drawPlate, engrave and the brushed look move out of CompressorPanel
  so both devices share them. The Knob takes an arc colour, because
  the orange arc vanished on the orange plate.
- No meter and no tap. Gain sits in every voice of an instrument, and
  a tap would add two audio nodes to each note for a display nobody
  asked for.

Found on the way:
- Knob.dialGlobal added half the dial unscaled. In a zoomed graph it
  pointed beside the dial. The Compressor's e2e only hit by luck.

Tests:
- Unit: 2 new for the Gain plate and its taper.
- e2e 34 on atx: ALL PASSED, 126 checks (1 new).
- Gate on atx: CHECK EXIT 0, 283 files, 4359 tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/soundfx/nodes.ts

@@ -535,7 +535,7 @@ export const SoundFxOutput: SoundFxNodeDef = {
   emit: arriving,
 };
 
-const GAIN = numberRow('gain', 'Gain (dB)', -120, 24, 0.5);
+export const GAIN_ROW = numberRow('gain', 'Gain (dB)', -120, 24, 0.5);
 
 /** How an effect with a Bypass and a Wet says so, at the end of its description (#354). */
 const BLENDS =
@@ -549,11 +549,11 @@ export const SoundFxGain: SoundFxNodeDef = effect(
     description:
       'Makes the sound louder or quieter, in decibels: 0 leaves it as it is, -6 is about half as loud, +6 twice. Above 0 it can clip.' +
       BLENDS,
-    inputs: [audioIn, followingIn(GAIN)],
+    inputs: [audioIn, followingIn(GAIN_ROW)],
     outputs: [audioOut],
     params: [],
     emit: (sounds, amounts) => {
-      const db = held(amounts.gain, GAIN);
+      const db = held(amounts.gain, GAIN_ROW);
       return withStages(arriving(sounds), isNone(db) ? [] : [{ kind: 'gain', db }]);
     },
   },