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/generate_blood_mask.py

@@ -0,0 +1,15 @@
+"""Deterministic original blood decal mask (not a Valve asset)."""
+from PIL import Image, ImageDraw, ImageFilter
+from pathlib import Path
+import random
+rng=random.Random(41)
+im=Image.new('RGBA',(512,512))
+d=ImageDraw.Draw(im)
+for i in range(120):
+ x=rng.gauss(256,70); y=rng.gauss(256,65)
+ radius=rng.uniform(4,48) if i<55 else rng.uniform(2,8)
+ d.ellipse((x-radius,y-radius,x+radius,y+radius),fill=(100+rng.randrange(30),5,8,rng.randrange(155,240)))
+im=im.filter(ImageFilter.GaussianBlur(0.8))
+p=Path(__file__).resolve().parents[1]/'assets/effects/blood.png'
+p.parent.mkdir(parents=True,exist_ok=True)
+im.save(p)