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/editor/debug/graphHooks.ts

@@ -4,6 +4,7 @@ import { soundLibrary } from '../../viewport/SoundLibrary';
 import type { Widget } from '../../ui';
 import { currentNodePalette } from '../graph/NodePalette';
 import { CompressorPanel } from '../soundfx/CompressorPanel';
+import { GainPanel } from '../soundfx/GainPanel';
 import { EqPanel } from '../soundfx/EqPanel';
 import { rectOf } from '../textProbe';
 import { centreOf, snapButton, type HookContext } from './hookHelpers';
@@ -243,6 +244,34 @@ export function graphHooks({ state, editor }: HookContext) {
         };
       },
     },
+    /**
+     * The panel a Gain node shows in the sound FX graph (#384): whether it is shown and where, its
+     * knob, and the node's ports.
+     */
+    gain: {
+      panel: (nodeId: string) => {
+        const canvas = editor.soundFxEditor.canvas;
+        const panel = canvas.faceOf(nodeId)?.widget;
+        if (!(panel instanceof GainPanel)) return null;
+        const node = canvas.nodeScreen(nodeId);
+        const ports = (keys: string[], isOutput: boolean) =>
+          Object.fromEntries(keys.map((key) => [key, canvas.portGlobal(nodeId, key, isOutput)]));
+        return {
+          shown: shownOnScreen(panel),
+          rect: screenRect(panel),
+          size: { w: panel.w, h: panel.h },
+          node: node?.rect ?? null,
+          inputs: ports(node?.inputs ?? [], false),
+          outputs: ports(node?.outputs ?? [], true),
+          knob: {
+            value: panel.knob.value,
+            enabled: panel.knob.enabled,
+            number: panel.knob.shown,
+            dial: panel.knob.dialGlobal(),
+          },
+        };
+      },
+    },
     instrument: {
       current: () => editor.instrumentEditor.currentAssetId,
       info: () => editor.instrumentEditor.shownInfo,