Add industrial metal soundtrack with seamless loop

- assets/audio/music/zombie_industrial_loop.ogg: 57.60 s Ogg Vorbis loop
  generated locally with YuE2-3B on the Tesla V100 (score written for this
  track; vocals removed with HTDemucs). The loop is spliced at the track's
  structural repeat with a 0.6 s crossfade: measured seam dip <= 1.6 dB.
- scripts/main.gd: "Music" AudioStreamPlayer at -11 dB, started at boot on
  desktop and on the start click on Web (browser autoplay policy). It forces
  stream.loop at runtime because the Web staging step re-imports the OGG with
  loop=false.
- tools/test_music.gd: soundtrack test (loop, length, volume, playback,
  still playing after player death).
- docs/MUSIC.md: source, loop construction, verification, licence.
- docs/WEB-DEPLOY.md: release ab6054111f5919a8 notes.

Initial commit of the project.

tools/package_web_release.py

@@ -0,0 +1,33 @@
+"""Immutable content-addressed Web releases with offline gzip compression."""
+from pathlib import Path
+import hashlib, gzip, json, shutil, os
+root=Path(__file__).resolve().parents[1]
+source=root/'build/web'
+output=root/'build/web-deploy'
+files=sorted(p for p in source.iterdir() if p.is_file() and not p.name.endswith(('.gz','.br')))
+identity=hashlib.sha256()
+for p in files:
+    identity.update(p.name.encode()+b'\0'+p.read_bytes())
+version=identity.hexdigest()[:16]
+if output.exists(): shutil.rmtree(output)
+release=output/'releases'/version
+release.mkdir(parents=True)
+report=[]
+for p in files:
+    destination=release/p.name
+    shutil.copyfile(p,destination)
+    raw=p.read_bytes()
+    if p.suffix in ['.wasm','.pck','.js','.html']:
+        compressed=gzip.compress(raw,compresslevel=9,mtime=0)
+        assert gzip.decompress(compressed)==raw
+        z=destination.with_name(destination.name+'.gz')
+        z.write_bytes(compressed)
+        os.utime(z,(destination.stat().st_mtime,destination.stat().st_mtime))
+        report.append({'file':p.name,'original':len(raw),'gzip':len(compressed),'sha256':hashlib.sha256(raw).hexdigest()})
+html=(source/'index.html').read_text()
+html=html.replace('<head>','<head>\n\t\t<base href="/pub/zombie-fps/releases/'+version+'/">',1)
+(output/'index.html').write_text(html)
+(root/'build/web-compression-report.json').write_text(json.dumps({'version':version,'files':report},indent=2)+'\n')
+print('Release:',version)
+for r in report: print(r['file'],r['original'],'->',r['gzip'])
+print('Total:',sum(r['original'] for r in report),'->',sum(r['gzip'] for r in report))