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/blood_effects.gd

@@ -0,0 +1,66 @@
+extends RefCounted
+const MAX_STAINS := 160
+
+static func impact(world: Node3D, point: Vector3, direction: Vector3) -> void:
+	# CPU particles work in both Forward+ and WebGL Compatibility.
+	var particles := CPUParticles3D.new()
+	particles.name = "BloodSpray"
+	particles.amount = 28
+	particles.lifetime = 0.65
+	particles.one_shot = true
+	particles.explosiveness = 1.0
+	particles.direction = direction.normalized()
+	particles.spread = 65
+	particles.initial_velocity_min = 1.3
+	particles.initial_velocity_max = 3.5
+	particles.gravity = Vector3(0, -9.8, 0)
+	particles.scale_amount_min = 0.5
+	particles.scale_amount_max = 1.5
+	var mesh := SphereMesh.new()
+	mesh.radius = 0.018
+	mesh.height = 0.036
+	var material := StandardMaterial3D.new()
+	material.albedo_color = Color(0.45, 0.015, 0.025)
+	material.roughness = 0.35
+	mesh.material = material
+	particles.mesh = mesh
+	world.add_child(particles)
+	particles.global_position = point
+	particles.emitting = true
+	world.get_tree().create_timer(1.3).timeout.connect(particles.queue_free)
+	var query := PhysicsRayQueryParameters3D.create(point + Vector3.UP * 0.05, point + Vector3.DOWN * 8, 1)
+	var hit := world.get_world_3d().direct_space_state.intersect_ray(query)
+	if hit.is_empty() or hit.normal.dot(Vector3.UP) < 0.7:
+		return
+	var width := randf_range(0.55, 1.15)
+	var stain: Node3D
+	if RenderingServer.get_current_rendering_method() == "gl_compatibility":
+		# WebGL has no projected decals: textured horizontal patch on floor.
+		var patch := MeshInstance3D.new()
+		var plane := PlaneMesh.new()
+		plane.size = Vector2(width, width)
+		var mat := StandardMaterial3D.new()
+		mat.albedo_texture = load("res://assets/effects/blood.png")
+		mat.transparency = BaseMaterial3D.TRANSPARENCY_ALPHA
+		mat.depth_draw_mode = BaseMaterial3D.DEPTH_DRAW_DISABLED
+		mat.roughness = 0.9
+		plane.material = mat
+		patch.mesh = plane
+		patch.cast_shadow = GeometryInstance3D.SHADOW_CASTING_SETTING_OFF
+		stain = patch
+	else:
+		var decal := Decal.new()
+		decal.texture_albedo = load("res://assets/effects/blood.png")
+		decal.size = Vector3(width, 0.25, width)
+		decal.cull_mask = 1
+		decal.upper_fade = 0
+		decal.lower_fade = 0
+		stain = decal
+	stain.name = "BloodStain"
+	world.add_child(stain)
+	stain.global_position = hit.position + Vector3.UP * 0.012
+	stain.rotation.y = randf_range(0, TAU)
+	stain.add_to_group("blood_stains")
+	var stains := world.get_tree().get_nodes_in_group("blood_stains")
+	if stains.size() > MAX_STAINS:
+		stains[0].queue_free()