Play a sound backwards with a Reverse switch on the asset (#321)

A sound asset can now play its stretch backwards. The switch sits on
the asset, so every way the sound is heard plays it reversed: Play
sound, a song item, an instrument's Sample and a Pitch in a sound FX.
There is no Reverse node in the sound FX graph. The user dropped it,
because playing backwards needs the whole sound in advance, which a
bus or a Chip does not have.

- The cut gets a `reverse` flag, stored as the asset property
  `reverse` (default off). The cut name ends in `~reverse` when it is
  on, so a reversed sound over the whole file is a cut of its own.
- The handles keep marking the same part of the file. The reversed
  copy is made once, like any cut, and the fades follow what is heard:
  Fade in ramps at the end handle, Fade out at the start handle.
- The Sound tab has a toggle with a left-pointing play triangle. Its
  playhead runs from right to left, and the wave draws the ramps at
  the edges they now belong to. The inspector has a Reverse row.
- The icon is a tab header icon (`HEADER_ICON_SVG`, next to the snap
  magnet), not a top bar icon. The first gate run had it in the top
  bar set, and the test that fixes that set went red.
- A song item draws the waveform of a reversed sound backwards.
  Lengths stay as they are.
- The header label reads "Fade in" instead of "Fade in (ms)" and the
  fade fields are 8 px narrower. With the new button the readout was
  cut to "0.03 s of 0..." at 1280 px, now it shows in full. The
  inspector keeps the unit.

Tests:
- Unit sounds 22, sound-library 85 (1 new each). Counter-checks:
  without turning the samples round, without moving the fade in to
  the end, and without mirroring the peaks, all red.
- e2e 34: ALL PASSED, 72 checks (2 new). The reversed stretch starts
  with the last samples of the forward one, and the playhead ran
  0.75, 0.60, 0.45, 0.30, 0.15 over the whole file.
- Gate: first run red, 1 of 4031 (the top bar icon list). After
  moving the icon, CHECK EXIT 0, 260 files, 4031 tests.

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

tools/e2e/34-sound.mjs

@@ -1682,6 +1682,46 @@ export async function sound(t) {
       (await json('__gm.soundTab.playhead()')) === null,
     JSON.stringify({ last: tabHeard.last, tabPlayhead, tabEnded }),
   );
+  // 34h-iii. Reverse plays the same stretch backwards, the playhead running from its end (#321).
+  const reverseButton = await json('__gm.soundTab.reverseButton()');
+  await click(reverseButton.x, reverseButton.y);
+  await sleep(150);
+  const reversed = await json(`__gm.soundTab.played(${JSON.stringify(droneId)})`);
+  const reversedSamples = await evaluate(`__gm.soundTab.samples(${JSON.stringify(reversed)})`);
+  check(
+    'Reverse turns the stretch around: its first samples are the last of the stretch played forwards',
+    (await droneProps())?.reverse === true &&
+      (await json('__gm.soundTab.reverseButton()')).on === true &&
+      reversed === `${unfaded}~reverse` &&
+      reversedSamples?.length === rawSamples?.length &&
+      reversedSamples?.head.every((v, i) => v === rawSamples?.tail.at(-1 - i)),
+    JSON.stringify({ reversed, head: reversedSamples?.head, tail: rawSamples?.tail }),
+  );
+  // The whole file (0.75 s) gives the playhead time for several readings.
+  const tabWhole = await json('__gm.soundTab.wholeButton()');
+  await click(tabWhole.x, tabWhole.y);
+  await sleep(150);
+  await click(tabPlay.x, tabPlay.y);
+  await waitFor(
+    () => json('__gm.soundTab.playhead()'),
+    (seconds) => seconds !== null,
+    10,
+  );
+  const backwards = (
+    await trace(
+      () => json('__gm.soundTab.playhead()'),
+      (seconds) => seconds === null,
+    )
+  ).filter((seconds) => seconds !== null);
+  check(
+    'played backwards, the playhead runs from the end of the file towards its start',
+    (await droneProps())?.reverse === true &&
+      backwards.length >= 2 &&
+      backwards.every((seconds) => seconds >= 0 && seconds <= 0.75) &&
+      backwards.every((seconds, i) => i === 0 || seconds <= backwards[i - 1]) &&
+      backwards.at(-1) < backwards[0],
+    JSON.stringify(backwards),
+  );
   await shot('34-sound-tab');
 
   // 34i. The step takes back everything it made, so the steps after it see the fixture again.