Load the sounds of a project ahead of their first play

A sound was fetched and decoded on its first Play sound. That play
started late, its beginning was not heard, and a graph that tuned the
voice in the meantime had moved on by the time it started. Images and
fonts were already loaded when a project opens. Sounds now are too.

- projectFiles lists the sounds of a project, and preloadProjectFiles
  hands them to the new soundLibrary.preload. That covers the editor
  and the published game.
- Preloading opens no audio device. It decodes with an
  OfflineAudioContext at 48 kHz. The real AudioContext is still made
  on the first play, and a source resamples where the device runs at
  another rate.
- A run started right after opening a project can still find a sound
  loading. It then starts late as before.

Tests:
- sound-library: a preload makes only the offline context, and a
  play after it starts at once, opening the device only then.
- sound-library + project-files: 18 tests, green.
- e2e 34 on melektaus-atx: ALL PASSED, 30 checks.
- Counter-check: decoding with the real AudioContext again fails the
  new test.

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

tests/viewport/sound-library.test.ts

@@ -194,6 +194,39 @@ describe('the sound library', () => {
     expect(sources.filter((s) => s.started)).toHaveLength(2);
   });
 
+  it('loads ahead of the first play without opening the device, so that play starts at once', async () => {
+    const opened: string[] = [];
+    vi.stubGlobal(
+      'AudioContext',
+      class {
+        constructor() {
+          opened.push('device');
+          return fakeContext();
+        }
+      },
+    );
+    vi.stubGlobal(
+      'OfflineAudioContext',
+      class {
+        constructor() {
+          opened.push('offline');
+          return fakeContext();
+        }
+      },
+    );
+    const library = new SoundLibrary();
+    library.preload([FILE, OTHER, '']);
+    await settle();
+    await arrive();
+    expect(opened).toEqual(['offline']);
+    expect(library.stats()).toMatchObject({ decoded: 2, started: 0 });
+    // No await: the voice runs in the frame that asked for it, not once the bytes are in.
+    const voice = library.play(FILE, 1, false);
+    expect(library.stats()).toMatchObject({ started: 1, playing: 1 });
+    expect(library.tune(voice, {})).toBe(true);
+    expect(opened).toEqual(['offline', 'device']);
+  });
+
   it('remembers what it last started, so a muted check can see it at all', async () => {
     const library = new SoundLibrary();
     library.play(FILE, 0.25, true);