Zoom the height of a sound's wave over its level scale (#357)

The Sound tab always drew the wave at the full height of the tab. Quiet
stretches were hard to see, and there was no room above a loud wave.

- A level scale in dB stands left of the wave, a mark every 6 dB down
  from 0, as long as a mark lies inside the area and at least 14 px
  from the middle.
- The wheel over the scale zooms the height, from 0.25x to 16x. Over
  the wave it zooms the time as before.
- What is louder than the area stops at its edge. The fade ramps grow
  with the wave.
- Fit puts the height back to 1x. The tab keeps the height with its
  view when another tab is shown.
- The rules live in waveView.ts (zoomLevel, levelMarks), the hooks
  give the height, the scale and the top of a full-scale wave.

Tests:
- Unit wave-view: 10 green (2 new), 2 counter-checks each red
- e2e 34 (melektaus-atx): ALL PASSED, 87 checks (1 new)
- Gate (melektaus-atx): CHECK EXIT 0, 270 files, 4183 tests

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

src/editor/debug/soundHooks.ts

@@ -16,6 +16,11 @@ export function soundHooks({ state, editor }: HookContext) {
       /** The stretch between the handles, in seconds; null while the file is not there. */
       span: () => soundEditor.canvas.shownSpan(),
       duration: () => soundEditor.canvas.fileDuration(),
+      /** The height zoom (#357): the factor, the middle of the level scale, and the top of a full-scale wave. */
+      level: () => soundEditor.canvas.levelZoom,
+      scale: () => soundEditor.canvas.scaleCentre(),
+      fullScaleTop: () => soundEditor.canvas.fullScaleTop(),
+      fitButton: () => centreOf(soundEditor.fitButton),
       cursor: () => soundEditor.canvas.cursor,
       playhead: () => soundEditor.canvas.playhead,
       playing: () => soundEditor.isPlaying,