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/extract_textures.py

@@ -0,0 +1,40 @@
+"""Extract local Half-Life assets; never alters the Steam installation."""
+import pathlib, struct
+from PIL import Image
+PROJECT = pathlib.Path(__file__).resolve().parents[1]
+SOURCE = pathlib.Path.home()/'.steam/steam/steamapps/common/Half-Life/valve'
+out = PROJECT/'assets/textures'
+out.mkdir(parents=True, exist_ok=True)
+body = bytearray((SOURCE/'models/zombie.mdl').read_bytes())
+tex = bytearray((SOURCE/'models/zombiet.mdl').read_bytes())
+base = len(body)
+count, records, data, refs, families, skins = struct.unpack_from('<6i', tex, 180)
+for i in range(count):
+    pos = records + i*80
+    name, flags, w, h, pixel = struct.unpack_from('<64s4i', tex, pos)
+    image = Image.frombytes('P', (w,h), bytes(tex[pixel:pixel+w*h]))
+    image.putpalette(bytes(tex[pixel+w*h:pixel+w*h+768]))
+    image.convert('RGB').save(out/('zombie_%02d.png'%i))
+    struct.pack_into('<i', tex, pos+76, pixel+base)
+struct.pack_into('<6i', body, 180, count, records+base, data+base, refs, families, skins+base)
+body.extend(tex)
+struct.pack_into('<i', body, 72, len(body))
+(PROJECT/'assets/zombies/zombie_textured.mdl').write_bytes(body)
+wad = (SOURCE/'halflife.wad').read_bytes()
+n, directory = struct.unpack_from('<ii',wad,4)
+wanted = {'LAB1_W3':'wall.png','C1A0_LABFLR':'floor.png','C1A0_C1':'ceiling.png'}
+found = set()
+for i in range(n):
+    offset, _, _, typ, compression, _, _, raw = struct.unpack_from('<iiiBBBB16s',wad,directory+32*i)
+    name = raw.split(b'\0')[0].decode().upper()
+    if name not in wanted or compression:
+        continue
+    _, w,h, a,b,c,d = struct.unpack_from('<16s6I',wad,offset)
+    palette_offset = offset+d+(w//8)*(h//8)+2
+    image = Image.frombytes('P',(w,h),wad[offset+a:offset+a+w*h])
+    image.putpalette(wad[palette_offset:palette_offset+768])
+    image.convert('RGB').save(out/wanted[name])
+    found.add(name)
+    print(name,w,h)
+assert found == set(wanted), ('Missing WAD textures',set(wanted)-found)
+print('Zombie textures:',count)