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/debug/graphHooks.ts

@@ -274,12 +274,13 @@ export function graphHooks({ state, editor }: HookContext) {
       },
     },
     /**
-     * The panel a Gain node shows (#384), and a Pitch or a Mix (#390): whether it is shown and where,
+     * The panel a Gain node shows (#384), a Pitch or a Mix (#390), a Pan (#392): whether it is shown and where,
      * its knob, the Mix's lever and the node's ports.
      */
     gain: { panel: (nodeId: string) => knobPanel(nodeId) },
     pitch: { panel: (nodeId: string) => knobPanel(nodeId) },
     mix: { panel: (nodeId: string) => knobPanel(nodeId) },
+    pan: { panel: (nodeId: string) => knobPanel(nodeId) },
     /**
      * The panel a Chip node shows in the instrument graph (#386): whether it is shown and where, its
      * knobs, which settings are idle, the picked wave, the lever, both screens and the node's ports.