Let the Reverb's screen ring out and its reflections drift (#401)

The round screen of the Reverb stood still. It now moves, only for
the look of it, and says nothing it did not say before.

- The sound rings out again and again in real time. The dot in the
  middle flashes, and a thin front runs out over the rings as fast as
  the tail does. It fades as the tail falls, then 0.8 s of quiet.
- A reflection lights up for a moment once the front has passed it.
- The reflections turn slowly round the dot, each at its own pace and
  way (at most 0.16 rad/s). They sway 1.5 px in and out and twinkle.
  Their drift comes from a noise of its own, so they sit where they
  always did.
- Rings, grid and dashed end stay one drawing. Only the dots, the
  front and the flash are drawn again each frame.
- Nothing moves without a tail to see: no sound picked, or wet at 0.

Tests:
- reverb-view: 13 tests green (3 new)
- e2e 34 on melektaus-atx: ALL PASSED, 144 checks
- gate on melektaus-atx: 293 files, 4498 tests

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

tools/e2e/34-sound.mjs

@@ -1215,6 +1215,7 @@ export async function sound(t) {
   // #389: a Reverb node is drawn as a device with a round screen of its tail. A knob its type does
   //       not take goes over to Custom with the type's numbers, as one undo step; a sound picked on
   //       it makes the type Sound and the screen draws that sound's tail. It is wired to nothing.
+  // #401: the screen moves by itself.
   await mutate(`
     const graph = __gm.state.project.assets.find((a) => a.id === ${JSON.stringify(fxId)}).graph;
     graph.nodes.push(${JSON.stringify(node('fx-rv', 'SoundFxReverb', 0, 400, { room: 'hall' }))});`);
@@ -1226,6 +1227,8 @@ export async function sound(t) {
       `__gm.state.project.assets.find((a) => a.id === ${JSON.stringify(fxId)}).graph.nodes.find((n) => n.id === 'fx-rv').params`,
     );
   const rvShown = await reverbPanel();
+  await sleep(300);
+  const rvLater = await reverbPanel();
   const rvRect = rvShown.rect;
   const rvBeside = (p) => p.y > rvRect.y && p.y < rvRect.y + rvRect.h;
   const rvDial = rvShown.knobs.decay.dial;
@@ -1261,6 +1264,8 @@ export async function sound(t) {
       rvShown.scope.reach < 0.5 &&
       rvShown.scope.says.ends === 'ENDS 2.52 s' &&
       rvShown.highs === 'HIGHS 5.5 kHz' &&
+      rvLater.scope.seconds > rvShown.scope.seconds &&
+      rvLater.scope.at !== rvShown.scope.at &&
       rvTurned.params.room === 'custom' &&
       rvTurned.params.decay > 2.5 &&
       rvTurned.params.damping === 0.35 &&
@@ -1280,7 +1285,7 @@ export async function sound(t) {
       rvSound.panel.scope.rings > 0 &&
       rvSoundUndone.room === 'cave' &&
       rvSoundUndone.impulse === undefined,
-    JSON.stringify({ rvShown, rvTurned, rvUndone, rvCave, rvOffered, rvSound, rvSoundUndone }),
+    JSON.stringify({ rvShown, rvLater, rvTurned, rvUndone, rvCave, rvOffered, rvSound, rvSoundUndone }),
   );
   await mutate(`
     const graph = __gm.state.project.assets.find((a) => a.id === ${JSON.stringify(fxId)}).graph;