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/prepare_web.py

@@ -0,0 +1,15 @@
+"""Build a GDExtension-free staging project; leaves desktop import plugin intact."""
+from pathlib import Path
+import shutil
+root = Path(__file__).resolve().parents[1]
+stage = root/'build/web-stage'
+if stage.exists():
+    shutil.rmtree(stage)
+stage.mkdir(parents=True)
+for directory in ['scripts','scenes','assets']:
+    shutil.copytree(root/directory,stage/directory,ignore=shutil.ignore_patterns('*.mdl','*.import','*.uid','half_life_manifest.json','p_9mmar.scn','w_9mmar.scn'))
+config=(root/'project.godot').read_text().replace('enabled=PackedStringArray("res://addons/goldsrc/plugin.cfg")','enabled=PackedStringArray()')
+config=config.replace('renderer/rendering_method="forward_plus"','renderer/rendering_method="gl_compatibility"')
+(stage/'project.godot').write_text(config)
+shutil.copyfile(root/'export_presets.cfg',stage/'export_presets.cfg')
+print(stage)