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>

tests/editor/wave-view.test.ts

@@ -8,6 +8,9 @@ import {
   draggedSpan,
   edgeAt,
   fitWave,
+  levelMarks,
+  MAX_LEVEL,
+  MIN_LEVEL,
   rulerLabel,
   rulerStep,
   secondsToX,
@@ -16,9 +19,32 @@ import {
   storedCut,
   wavePeaks,
   xToSeconds,
+  zoomLevel,
   zoomWave,
 } from '../../src/editor/sound/waveView';
 
+describe('the height of the wave (#357)', () => {
+  it('zooms between a quarter and sixteen times full scale', () => {
+    expect(zoomLevel(1, 2)).toBe(2);
+    expect(zoomLevel(1, 0.5)).toBe(0.5);
+    expect(zoomLevel(0.3, 0.5)).toBe(MIN_LEVEL);
+    expect(zoomLevel(12, 2)).toBe(MAX_LEVEL);
+  });
+
+  it('marks every 6 dB that lies inside the area and not too close to the middle', () => {
+    // Level 1, full scale 100 px from the middle: 0 dB at 100, -6 at 50, -12 at 25, -18 at 12.6.
+    const marks = levelMarks(1, 100, 110, 14);
+    expect(marks.map((m) => m.db)).toEqual([0, -6, -12]);
+    expect(marks[1]!.offset).toBeCloseTo(50.1, 1);
+    // Zoomed out, -6 dB lies 12.5 px from the middle and is left out.
+    expect(levelMarks(0.25, 100, 110, 14).map((m) => m.db)).toEqual([0]);
+    // Zoomed in, 0 dB lies past the edge and the quiet ones show.
+    const loud = levelMarks(16, 100, 110, 14).map((m) => m.db);
+    expect(loud[0]).toBe(-24);
+    expect(loud).not.toContain(0);
+  });
+});
+
 describe('the wave view', () => {
   it('zooms about the pointer and fits the file with a margin either side', () => {
     const view = { panX: 10, pxPerSecond: 100 };