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/sound/waveView.ts

@@ -49,6 +49,43 @@ export function clampWave(view: WaveView, duration: number, width: number): Wave
   return { ...view, panX: clamp(view.panX, least, lead) };
 }
 
+/** How far the wave's height zooms: out to a quarter, in to a sixteenth of full scale (#357). */
+export const MIN_LEVEL = 0.25;
+export const MAX_LEVEL = 16;
+/** Decibels apart the level scale's marks are. */
+const LEVEL_STEP_DB = 6;
+/** The quietest mark the scale goes down to. */
+const QUIETEST_DB = -96;
+
+export function zoomLevel(level: number, factor: number): number {
+  return clamp(level * factor, MIN_LEVEL, MAX_LEVEL);
+}
+
+/** Pixels from the middle of the wave an amplitude is drawn at, `half` being full scale at level 1. */
+export function levelOffset(amplitude: number, level: number, half: number): number {
+  return amplitude * level * half;
+}
+
+/**
+ * The marks of the level scale: every 6 dB down from 0, as far from the middle as they are drawn.
+ * Only those within `room` of the middle and at least `minPx` from it – 6 dB halve the distance,
+ * so that keeps them as far apart from each other too.
+ */
+export function levelMarks(
+  level: number,
+  half: number,
+  room: number,
+  minPx: number,
+): { db: number; offset: number }[] {
+  const marks: { db: number; offset: number }[] = [];
+  for (let db = 0; db >= QUIETEST_DB; db -= LEVEL_STEP_DB) {
+    const offset = levelOffset(10 ** (db / 20), level, half);
+    if (offset < minPx) break;
+    if (offset <= room) marks.push({ db, offset });
+  }
+  return marks;
+}
+
 /** Seconds between the ruler's labels: 1, 2 or 5 times a power of ten, at least `minPx` apart. */
 export function rulerStep(pxPerSecond: number, minPx: number): number {
   const rough = minPx / pxPerSecond;