Show a Pan node as a one-knob device in cyan (#392)

After the Gain, the Pitch and the Mix, the Pan is the next node that
is just one number. It now gets the same device in the graph instead
of a row for its pan.

- PAN_DEVICE: the Gain's 160x160 plate in cyan with one bipolar knob
  for Pan. The middle stands straight up, and the turn goes evenly to
  L and R, which are engraved at its ends like A and B on the Mix.
- PAN_TAPER in hardwareView is 1, so left and right are heard alike.
  Device and unit test read the same constant.
- PAN_ROW is exported from the core, so the knob has the node's range.
- A linked Pan greys the knob, as for the Gain.
- The test hook __gm.pan.panel returns the same shape as the Gain's.
- No instrument preset uses a Pan, so no preset columns change.

Tests:
- Unit hardware-view: 17 tests, 2 new (the turn, and L and R inside
  the plate and clear of the screws). Counter-check: PAN_TAPER 2
  turns the turn test red.
- e2e 34 (melektaus-atx): 135 checks, 1 new.
- Gate (melektaus-atx): 287 files, 4399 tests.
- Pictures looked at: Gain, Pitch, Mix and Pan in a row, Pan at -0.35.

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

src/editor/soundfx/nodeFaces.ts

@@ -15,10 +15,10 @@ import { EnvelopeFace } from './EnvelopeFace';
 import { EqPanel } from './EqPanel';
 import { FilterPanel } from './FilterPanel';
 import { KnobFace } from './KnobFace';
-import { GAIN_DEVICE, MIX_DEVICE, PITCH_DEVICE } from './KnobPanel';
+import { GAIN_DEVICE, MIX_DEVICE, PAN_DEVICE, PITCH_DEVICE } from './KnobPanel';
 import { ReverbFace } from './ReverbFace';
 
-/** The nodes drawn as a face of their own instead of rows (#380, #381, #384–#386, #388–#390), by type. */
+/** The nodes drawn as a face of their own instead of rows (#380, #381, #384–#386, #388–#390, #392), by type. */
 export function nodeFaceFor(state: EditorState, assetId: string, node: GraphNode): NodeFace | null {
   if (node.type === InstrumentChip.type) return new ChipFace(state, assetId, node.id);
   if (node.type === InstrumentEnvelope.type) return new EnvelopeFace(state, assetId, node.id);
@@ -26,7 +26,9 @@ export function nodeFaceFor(state: EditorState, assetId: string, node: GraphNode
   if (node.type === SoundFxParametricEq.type)
     return new CurveFace(new EqPanel(state, true), assetId, node.id);
   if (node.type === SoundFxFilter.type) return new CurveFace(new FilterPanel(state, true), assetId, node.id);
-  const knobbed = [GAIN_DEVICE, PITCH_DEVICE, MIX_DEVICE].find((device) => device.type === node.type);
+  const knobbed = [GAIN_DEVICE, PITCH_DEVICE, MIX_DEVICE, PAN_DEVICE].find(
+    (device) => device.type === node.type,
+  );
   if (knobbed) return new KnobFace(state, assetId, node.id, knobbed);
   if (node.type === SoundFxReverb.type) return new ReverbFace(state, assetId, node.id);
   return null;