Snap the routing graph of a song with the header's magnet (#350)

The routing of a song is the same graph editor as a sound FX, but its
tab had no magnet, so its nodes could not be put on the grid.

- The Song tab carries the magnet next to Routing. It is off at first
  and acts on the routing graph.
- It is greyed out on the timeline, as Grid is greyed out in the
  routing. Switching the view only updates these two buttons, so a
  playing song is not handed its sequence again.
- The check hooks give the magnet and the snapping of the routing.

Tests:
- e2e 65 (melektaus-atx): ALL PASSED, 52 checks (1 new)
- Counter-check (magnet enabled on the timeline): exactly this check red
- Gate (melektaus-atx): CHECK EXIT 0, 270 files, 4178 tests

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

tools/e2e/65-song.mjs

@@ -890,9 +890,27 @@ export async function song(t) {
     song.tracks.forEach((track) => Object.assign(track, { mute: false, solo: false, fx: '' }));`);
   await evaluate(`__gm.state.openAsset(${JSON.stringify(id)})`);
   await sleep(200);
+  const magnetOnTimeline = await json('__gm.song.routing.snapButton()');
   p = await json("__gm.song.viewAt('routing')");
   await click(p.x, p.y);
   await sleep(300);
+  // The magnet is the routing's, as in every graph tab: off at first, greyed out on the timeline.
+  const magnet = await json('__gm.song.routing.snapButton()');
+  const snapOff = await json('__gm.song.routing.snapping()');
+  await click(magnet.x, magnet.y);
+  const snapOn = await json('__gm.song.routing.snapping()');
+  const lit = (await json('__gm.song.routing.snapButton()')).toggled;
+  await click(magnet.x, magnet.y);
+  check(
+    'the magnet in the header switches snapping in the routing on and off',
+    !magnetOnTimeline.enabled &&
+      magnet.enabled &&
+      snapOff === false &&
+      snapOn === true &&
+      lit &&
+      (await json('__gm.song.routing.snapping()')) === false,
+    JSON.stringify({ magnetOnTimeline, magnet, snapOff, snapOn, lit }),
+  );
   const routingOf = () => json(`__gm.song.routing.graph(${JSON.stringify(id)})`);
   // The tracks as they are now: more than in 65a.
   const ids = (await model()).tracks.map((track) => track.id);