Let solo win over the mute of the same track (#358)

A track with both M and S on stayed silent: the mute closed every send
before solo was asked. Pressing S on a muted track is meant to hear it.

- trackGates in routing.ts opens the sends of a soloed track even while
  it is muted. Its own items are heard as with solo alone.
- A muted track that a soloed one goes through, or that goes into it,
  stays silent as before.

Tests:
- Unit song-routing and song: 28 green (1 new), a counter-check red

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

tests/core/song-routing.test.ts

@@ -248,6 +248,15 @@ describe('what the buses of a routed song are set to (#350)', () => {
     );
   });
 
+  it('hears a soloed track even while it is muted', () => {
+    expect(heard(([a, ...rest]) => [{ ...a!, solo: true, mute: true }, ...rest])).toEqual({
+      a: ['1', 1],
+      b: ['1', 0],
+      c: ['1', 1],
+      d: ['0', 0],
+    });
+  });
+
   it('reads the sends of a song made before #350 as every track into the output', () => {
     const tracks = [{ ...newTrack(1), id: 'a' }];
     expect(songRoutes({ tracks })).toEqual([{ from: 'a', to: '', volume: 0, tap: 'post' }]);