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>

tests/editor/hardware-view.test.ts

@@ -1,7 +1,8 @@
 /**
  * The Compressor's hardware panel (#379): every part on the plate and none over another, the meter's
  * scale and needle, the LEDs, and the grain of the plate the same every time. The Gain's plate
- * (#384) and how its one knob turns, the Pitch's knob and the Mix's lever and ends (#390).
+ * (#384) and how its one knob turns, the Pitch's knob and the Mix's lever and ends (#390), the
+ * Pan's knob and ends on the Gain's plate (#392).
  */
 import { describe, expect, it } from 'vitest';
 import {
@@ -27,11 +28,12 @@ import {
   MIX_PANEL,
   needleAngle,
   needleStep,
+  PAN_TAPER,
   PANEL,
   PANEL_LAYOUT,
   PITCH_TAPER,
 } from '../../src/editor/soundfx/hardwareView';
-import { GAIN_ROW, PITCH_ROW } from '../../src/core/soundfx/nodes';
+import { GAIN_ROW, PAN_ROW, PITCH_ROW } from '../../src/core/soundfx/nodes';
 import { knobPosition } from '../../src/ui/knob';
 
 const overlap = (a: Box, b: Box) => a.x < b.x + b.w && b.x < a.x + a.w && a.y < b.y + b.h && b.y < a.y + a.h;
@@ -215,3 +217,34 @@ describe('the Pitch and the Mix (#390)', () => {
     expect(a.y).toBe(b.y);
   });
 });
+
+describe('the Pan (#392)', () => {
+  it('turns from the middle straight up, as far left as right, the whole turn for -1 to 1', () => {
+    const range = {
+      min: PAN_ROW.min ?? 0,
+      max: PAN_ROW.max ?? 0,
+      step: 0.01,
+      taper: PAN_TAPER,
+      bipolar: true,
+    };
+    expect(knobPosition(0, range)).toBeCloseTo(0.5, 5);
+    expect(knobPosition(0.5, range)).toBeCloseTo(0.75, 5);
+    expect(knobPosition(-0.5, range)).toBeCloseTo(0.25, 5);
+    expect(knobPosition(-1, range)).toBe(0);
+    expect(knobPosition(1, range)).toBe(1);
+  });
+
+  it('names L and R at the ends of the turn inside the Gain’s plate, clear of the screws', () => {
+    const spot = GAIN_LAYOUT.knob;
+    const { a, b } = knobEnds(spot);
+    for (const end of [a, b]) {
+      expect(end.x).toBeGreaterThan(0);
+      expect(end.x).toBeLessThan(GAIN_PANEL.w);
+      expect(end.y).toBeLessThan(GAIN_PANEL.h);
+      for (const screw of GAIN_LAYOUT.screws)
+        expect(Math.hypot(end.x - screw.x, end.y - screw.y)).toBeGreaterThan(14);
+    }
+    expect(a.x).toBeLessThan(spot.x);
+    expect(b.x).toBeGreaterThan(spot.x);
+  });
+});