feat(biome): abyssal bioluminescent + glow blocks + abyssal jellyfish + particles

- BlockDatabase: GLOW_CORAL_CYAN(10), GLOW_CORAL_VIOLET(11), LAVA_VENT(12) with emission data
- WorldGenerator: abyssal blocks at y<-40 (cyan 4%, violet 1%, vent 0.5% via 3D noise)
- Chunk.gd: multi-surface mesh (solid + 3 emissive surfaces with StandardMaterial3D emission)
- MobSpawner: abyssal jellyfish spawn when player y<-30, max 8
- AbyssalJellyfish: cyan fluo, slower, double damage (10), bioluminescent flash on attack
- BioluminescentParticles: 200 cyan-green particles follow player, only emit in abyss

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/ambience/BioluminescentParticles.gd

@@ -0,0 +1,88 @@
+extends GPUParticles3D
+
+# Bioluminescent floating particles — only active in abyss zone (player y < -30)
+
+const ABYSS_DEPTH_THRESHOLD: float = -30.0
+
+var _player: Node3D = null
+
+
+func _ready() -> void:
+	amount = 200
+	lifetime = 12.0
+	one_shot = false
+	explosiveness = 0.0
+	randomness = 1.0
+	visibility_aabb = AABB(Vector3(-30, -25, -30), Vector3(60, 50, 60))
+
+	var process_mat := ParticleProcessMaterial.new()
+	process_mat.emission_shape = ParticleProcessMaterial.EMISSION_SHAPE_BOX
+	process_mat.emission_box_extents = Vector3(28.0, 20.0, 28.0)
+
+	process_mat.direction = Vector3(0.0, 1.0, 0.0)
+	process_mat.spread = 120.0
+	process_mat.initial_velocity_min = 0.0
+	process_mat.initial_velocity_max = 0.08
+
+	process_mat.linear_accel_min = -0.03
+	process_mat.linear_accel_max = 0.03
+	process_mat.radial_accel_min = -0.02
+	process_mat.radial_accel_max = 0.02
+
+	process_mat.damping_min = 0.4
+	process_mat.damping_max = 0.9
+
+	process_mat.scale_min = 0.03
+	process_mat.scale_max = 0.12
+
+	# Cyan-green bioluminescent color
+	var grad := Gradient.new()
+	grad.set_color(0, Color(0.0, 0.8, 0.9, 0.0))
+	grad.set_color(1, Color(0.0, 0.8, 0.9, 0.0))
+	grad.add_point(0.1, Color(0.1, 0.9, 0.8, 0.7))
+	grad.add_point(0.85, Color(0.2, 0.7, 1.0, 0.6))
+	var grad_tex := GradientTexture1D.new()
+	grad_tex.gradient = grad
+	process_mat.color_ramp = grad_tex
+	process_mat.color = Color(0.1, 0.9, 0.8, 0.7)
+
+	process_material = process_mat
+
+	var quad := QuadMesh.new()
+	quad.size = Vector2(0.08, 0.08)
+
+	var mat := StandardMaterial3D.new()
+	mat.shading_mode = BaseMaterial3D.SHADING_MODE_UNSHADED
+	mat.transparency = BaseMaterial3D.TRANSPARENCY_ALPHA
+	mat.billboard_mode = BaseMaterial3D.BILLBOARD_ENABLED
+	mat.emission_enabled = true
+	mat.emission = Color(0.2, 0.8, 1.0)
+	mat.emission_energy_multiplier = 3.0
+	mat.albedo_color = Color(0.1, 0.9, 0.8, 0.7)
+
+	quad.surface_set_material(0, mat)
+	draw_pass_1 = quad
+
+	# Start inactive until we detect depth
+	emitting = false
+	_find_player()
+
+
+func _find_player() -> void:
+	var players := get_tree().get_nodes_in_group("player")
+	if players.size() > 0:
+		_player = players[0] as Node3D
+
+
+func _process(_delta: float) -> void:
+	if _player == null:
+		_find_player()
+		return
+
+	var in_abyss: bool = _player.global_position.y < ABYSS_DEPTH_THRESHOLD
+	if in_abyss != emitting:
+		emitting = in_abyss
+
+	# Follow player horizontally, float at player depth
+	if in_abyss:
+		global_position = _player.global_position