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.

docs/MUSIC.md

@@ -0,0 +1,67 @@
+# Soundtrack — industrial metal loop
+
+Endless survival loop played from the first frame, at `-11 dB`, independent of gameplay state.
+
+## Source
+
+Generated locally (2026-10-10) with **YuE2-3B Q8_0** through the audio.cpp CUDA build for the
+Tesla V100 (`~/dev/audio.cpp`, `cmake -DCMAKE_CUDA_ARCHITECTURES=70`), seed 31337, 16 steps,
+`cot=melody` + an external ABC score written for this track (K:Em Phrygian, `Q:1/4=168`,
+96 bars; the model rendered it at a perceived 112.5 BPM). Style prompt: instrumental
+industrial metal, Nine Inch Nails meets the Quake 2 soundtrack, machine drums, palm-muted
+downtuned guitars, atonal synth stabs, no vocals.
+
+`--lyrics "[instrumental]"` does **not** suppress singing (verified: male shouts 0:52-1:17 and
+1:37-2:35). The instrumental was therefore obtained by **HTDemucs** vocal removal
+(`--task sep --family htdemucs`, input resampled to 44.1 kHz, RTF 0.049) and remixed as
+`drums + bass + other`.
+
+Working files: `~/yue2-out/zombie-industrial/`.
+
+## Loop construction
+
+The track has a structural repeat (bars 24-30 ≈ 46-52, spectral similarity 0.95), so the loop
+splices at matching material instead of cutting arbitrarily:
+
+- region 51.37 s → 108.97 s of the instrumental master = 27 bars (bar = 2.1333 s, 112.5 BPM);
+- rotation by one tail segment plus a **0.60 s** equal-power crossfade;
+- result: **57.60 s**, seam dip ≤ 1.6 dB in 20 ms windows, sample step 0.04 (inaudible);
+- perceptual check on a 3 s + 3 s excerpt around the seam: no click, no level hole, natural
+  continuation.
+
+A longer variant (132 s) was rejected: its crossfade cancels (a -48 dB hole) because the two
+overlapped sections are out of phase.
+
+## Asset and import
+
+`assets/audio/music/zombie_industrial_loop.ogg` — Ogg Vorbis q6, 48 kHz stereo, 1.0 MB,
+-14 LUFS / -1.0 dBTP. Import: `loop=true`, `bpm=112.5`, `beat_count=108`, `bar_beats=4`.
+
+The Web staging step (`tools/prepare_web.py`) copies the asset but ignores `*.import`, and the
+staging import regenerates it with `loop=false`. `main.gd` therefore sets `stream.loop = true`
+at runtime: keep that line, it is what makes the browser build loop.
+
+## Runtime wiring
+
+`scripts/main.gd`: `_setup_music()` creates the `AudioStreamPlayer` named `Music` (group
+`music`), `_start_music()` starts it once, `_restart_music()` restarts it if the loop flag is
+ever absent. On Web the start is deferred to the click that captures the mouse, because
+browsers only allow audio after a user gesture. Music keeps playing while the player is dead;
+`R` reloads the scene and the track restarts with it.
+
+## Verification
+
+```
+godot --headless --path . --script res://tools/test_music.gd
+godot --display-driver headless --audio-driver PulseAudio --path . --script res://tools/test_music.gd
+godot --headless --path . -- --smoke
+```
+
+Both runs PASS: node present, `loop=true`, length 57.60 s, volume -11 dB, playback advancing
+(confirmed on the real PulseAudio device, not only the Dummy driver), music still playing after
+player death. Grenade, wave, death and damage-feedback tests also pass unchanged.
+
+## License
+
+YuE2-3B weights are **CC BY-NC 4.0** (non-commercial): this track is for personal/experimental
+use. It is an AI-generated render, not a recording of a third-party work.