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.

scripts/damage_feedback.gd

@@ -0,0 +1,90 @@
+extends CanvasLayer
+## Local-only trauma pivot: never modifies the player's aim angles.
+const SHAKE_SECONDS := 0.42
+const BLOOD_SECONDS := 1.15
+const SPLAT_COUNT := 7
+var pivot: Node3D
+var shake_left := 0.0
+var blood_left := 0.0
+var intensity := 0.0
+var phase := 0.0
+var active := true
+var overlay: Control
+var vignette: ColorRect
+var shader_material: ShaderMaterial
+var splats: Array[TextureRect] = []
+
+func _ready() -> void:
+	layer = 2
+	overlay = Control.new()
+	overlay.mouse_filter = Control.MOUSE_FILTER_IGNORE
+	add_child(overlay)
+	overlay.set_anchors_and_offsets_preset(Control.PRESET_FULL_RECT)
+	vignette = ColorRect.new()
+	vignette.mouse_filter = Control.MOUSE_FILTER_IGNORE
+	shader_material = ShaderMaterial.new()
+	shader_material.shader = load("res://assets/effects/damage_vignette.gdshader")
+	vignette.material = shader_material
+	overlay.add_child(vignette)
+	vignette.set_anchors_and_offsets_preset(Control.PRESET_FULL_RECT)
+	for i in SPLAT_COUNT:
+		var splat := TextureRect.new()
+		splat.texture = load("res://assets/effects/blood.png")
+		splat.expand_mode = TextureRect.EXPAND_IGNORE_SIZE
+		splat.stretch_mode = TextureRect.STRETCH_KEEP_ASPECT_CENTERED
+		splat.mouse_filter = Control.MOUSE_FILTER_IGNORE
+		splat.modulate.a = 0
+		overlay.add_child(splat)
+		splats.append(splat)
+
+func hit(amount: float) -> void:
+	if not active or amount <= 0:
+		return
+	# Refresh instead of stacking tweens/particles: bounded cost and shake.
+	intensity = minf(1.0, maxf(intensity, 0.55 + amount / 100.0) + 0.12)
+	shake_left = SHAKE_SECONDS
+	blood_left = BLOOD_SECONDS
+	phase = randf_range(0, TAU)
+	var viewport_size := get_viewport().get_visible_rect().size
+	for i in splats.size():
+		var splat := splats[i]
+		var size := minf(viewport_size.x, viewport_size.y) * randf_range(0.22, 0.38)
+		splat.size = Vector2.ONE * size
+		splat.pivot_offset = splat.size * 0.5
+		# Keep the central reticle and HUD readable; splats lie near the edges.
+		var center := Vector2.ZERO
+		match i % 4:
+			0: center = viewport_size * Vector2(randf_range(0.06, 0.17), randf_range(0.25, 0.90))
+			1: center = viewport_size * Vector2(randf_range(0.83, 0.95), randf_range(0.25, 0.90))
+			2: center = viewport_size * Vector2(randf_range(0.25, 0.75), randf_range(0.82, 0.98))
+			3: center = viewport_size * Vector2(randf_range(0.3, 0.8), randf_range(0.04, 0.13))
+		splat.position = center - splat.size * 0.5
+		splat.rotation = randf_range(-PI, PI)
+		splat.modulate = Color(0.85, 0.1, 0.12, 0.85 * intensity)
+	shader_material.set_shader_parameter("strength", intensity)
+
+func _process(delta: float) -> void:
+	if not active:
+		return
+	phase += delta * 42.0
+	shake_left = maxf(0, shake_left - delta)
+	blood_left = maxf(0, blood_left - delta)
+	var shake := pow(shake_left / SHAKE_SECONDS, 2.0) * intensity
+	if is_instance_valid(pivot):
+		pivot.position = Vector3(sin(phase) * 0.025, cos(phase * 1.3) * 0.018, sin(phase * 0.8) * 0.012) * shake
+		pivot.rotation = Vector3(sin(phase * 0.9) * 0.024, cos(phase) * 0.013, sin(phase * 1.1) * 0.035) * shake
+	var alpha := blood_left / BLOOD_SECONDS
+	for splat in splats:
+		splat.modulate.a = alpha * 0.85 * intensity
+	shader_material.set_shader_parameter("strength", minf(1, alpha * 1.4) * intensity)
+	if blood_left <= 0:
+		intensity = 0
+
+func stop() -> void:
+	active = false
+	shake_left = 0
+	blood_left = 0
+	intensity = 0
+	if is_instance_valid(pivot):
+		pivot.transform = Transform3D.IDENTITY
+	overlay.hide()