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/core/soundfx/nodes.ts

@@ -999,7 +999,7 @@ export const SoundFxFlanger: SoundFxNodeDef = effect(
   'own',
 );
 
-const PAN = numberRow('pan', 'Pan', -1, 1, 0.01);
+export const PAN_ROW = numberRow('pan', 'Pan', -1, 1, 0.01);
 
 export const SoundFxPan: SoundFxNodeDef = effect(
   {
@@ -1009,11 +1009,11 @@ export const SoundFxPan: SoundFxNodeDef = effect(
     description:
       'Moves the sound left or right: -1 is all left, 1 all right, 0 leaves it in the middle. A mono sound is spread on both sides and then moved. A stereo sound keeps the side it goes to, and the other side moves over into it: at 1 both sides are heard on the right. It can change while the sound plays, and in an instrument an Envelope can move it.' +
       BLENDS,
-    inputs: [audioIn, followingIn(PAN)],
+    inputs: [audioIn, followingIn(PAN_ROW)],
     outputs: [audioOut],
     params: [],
     emit: (sounds, amounts) => {
-      const pan = held(amounts.pan, PAN);
+      const pan = held(amounts.pan, PAN_ROW);
       return withStages(arriving(sounds), isNone(pan) ? [] : [{ kind: 'pan', pan }]);
     },
   },