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.

assets/effects/damage_vignette.gdshader

@@ -0,0 +1,8 @@
+shader_type canvas_item;
+render_mode unshaded;
+uniform float strength : hint_range(0.0, 1.0) = 0.0;
+void fragment() {
+	vec2 p = (UV - vec2(0.5)) * 2.0;
+	float edge = smoothstep(0.28, 1.12, length(p));
+	COLOR = vec4(0.65, 0.015, 0.025, edge * strength * 0.62);
+}