test(fixtures): replace synthetic media with real CC-BY/CC0 content

- Video: Big Buck Bunny (Blender Foundation, CC-BY 3.0) 8s segment
  replaces ffmpeg testsrc2 synthetic; container variants hero.{mov,webm,mkv,avi}
- Speech: macOS TTS (CC0) replaces unknown-origin speech files;
  format variants speech.{flac,ogg,m4a,aac,opus} for depth tests
- Chat images: Sharp SVG mock chat UI (CC0) replaces third-party
  app screenshots with personal content
- Registry: video.hero.{mov,webm,mkv,avi}, audio.speech.{flac,ogg,m4a,aac,opus}
- Integrity guard: 19 ffprobe assertions (was 8)
- Parity: 13177 passed, 0 dropped

tests/integration/fixtures/fixture-integrity.test.ts

@@ -23,7 +23,16 @@ describe("real fixtures decode through their real tools", () => {
 
   it.each([
     fixtures.video.hero.mp4,
+    fixtures.video.hero.mov,
+    fixtures.video.hero.webm,
+    fixtures.video.hero.mkv,
+    fixtures.video.hero.avi,
     fixtures.audio.speech.wav,
+    fixtures.audio.speech.flac,
+    fixtures.audio.speech.ogg,
+    fixtures.audio.speech.m4a,
+    fixtures.audio.speech.aac,
+    fixtures.audio.speech.opus,
     fixtures.audio.tagged,
   ])("ffprobe reads a positive duration from %s", (path) => {
     const out = execFileSync(