Drop the Gain's engraved name and centre its knob (#384)

The plate was taller than its one knob needs, and the engraved
"GAIN" only repeated the node's title. Without it the plate is
square, and the knob sits in its middle.

- The plate is 160x160 instead of 160x220, without the "GAIN"
  engraving.
- The knob is centred on what it shows: the "DB" caption and the
  dial. Since #382 its number hangs inside the dial's open bottom,
  so it adds no height.
- CAPTION_AIR (4) takes out the empty space above the caption's
  capitals. Without it the knob sat a little too low.

Measured on a picture at zoom 0.8: 19 px of air above "DB" and
19 px below the number. The user's screenshot showed about 14
above and 35 below.

Tests:
- Unit hardware-view: 12 tests green. The layout test now asks
  for the same air above and below.
- e2e 34 (melektaus-atx): ALL PASSED, 126 checks.
- Gate (melektaus-atx): CHECK EXIT 0, 283 files, 4359 tests.

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

tools/e2e/34-sound.mjs

@@ -1059,7 +1059,7 @@ export async function sound(t) {
     'a Gain node shows its panel with one knob between its ports; a drag turns it as one undo step, the wheel zooms the graph, a double-click resets it without a tab',
     gainShown.shown &&
       gainShown.size.w === 160 &&
-      gainShown.size.h === 220 &&
+      gainShown.size.h === 160 &&
       gainShown.knob.number === '-6' &&
       gainShown.knob.enabled &&
       Object.values(gainShown.inputs).length === 3 &&