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/mobs/AbyssalJellyfish.gd

@@ -0,0 +1,81 @@
+extends Jellyfish
+
+# Abyssal variant: slower, packs double damage, bioluminescent cyan flash on contact
+
+func _build_visuals() -> void:
+	# Cyan emissive dome material
+	var mat := StandardMaterial3D.new()
+	mat.transparency = BaseMaterial3D.TRANSPARENCY_ALPHA
+	mat.albedo_color = Color(0.1, 0.9, 1.0, 0.6)
+	mat.emission_enabled = true
+	mat.emission = Color(0.2, 0.8, 1.0)
+	mat.emission_energy_multiplier = 2.5
+	mat.cull_mode = BaseMaterial3D.CULL_DISABLED
+
+	_dome = MeshInstance3D.new()
+	var sphere := SphereMesh.new()
+	sphere.radius = 0.55
+	sphere.height = 0.75
+	_dome.mesh = sphere
+	_dome.material_override = mat
+	_dome.scale = Vector3(1.0, 0.7, 1.0)
+	add_child(_dome)
+
+	# Thin luminous tentacles
+	var tent_mat := StandardMaterial3D.new()
+	tent_mat.transparency = BaseMaterial3D.TRANSPARENCY_ALPHA
+	tent_mat.albedo_color = Color(0.1, 0.9, 1.0, 0.4)
+	tent_mat.emission_enabled = true
+	tent_mat.emission = Color(0.2, 0.8, 1.0)
+	tent_mat.emission_energy_multiplier = 2.0
+	tent_mat.cull_mode = BaseMaterial3D.CULL_DISABLED
+
+	var offsets: Array[Vector2] = [
+		Vector2(0.25, 0.0), Vector2(-0.25, 0.0),
+		Vector2(0.0, 0.25), Vector2(0.0, -0.25),
+		Vector2(0.18, 0.18), Vector2(-0.18, -0.18)
+	]
+	for off: Vector2 in offsets:
+		var tent := MeshInstance3D.new()
+		var cap := CapsuleMesh.new()
+		cap.radius = 0.02
+		cap.height = 0.9
+		tent.mesh = cap
+		tent.material_override = tent_mat
+		tent.position = Vector3(off.x, -0.65, off.y)
+		add_child(tent)
+		_tentacles.append(tent)
+
+	# Collision
+	var col := CollisionShape3D.new()
+	var cshape := SphereShape3D.new()
+	cshape.radius = 0.6
+	col.shape = cshape
+	add_child(col)
+
+
+func _ready() -> void:
+	# Override stats before parent _ready to ensure double damage and slower speed
+	damage = 10.0          # double base jellyfish damage
+	move_speed = 0.6       # slower
+	detection_radius = 8.0 # wider detection in dark abyss
+	super._ready()
+
+
+func _on_attack() -> void:
+	# Bioluminescent flash: briefly boost emission on dome
+	if _dome != null and _dome.material_override != null:
+		var mat := _dome.material_override as StandardMaterial3D
+		if mat != null:
+			mat.emission_energy_multiplier = 8.0
+			await get_tree().create_timer(0.15).timeout
+			if is_instance_valid(self) and _dome != null and _dome.material_override != null:
+				mat.emission_energy_multiplier = 2.5
+
+
+func _physics_process(delta: float) -> void:
+	var was_attacking: bool = (_state == State.ATTACK)
+	super._physics_process(delta)
+	# Trigger flash when we just entered attack
+	if not was_attacking and _state == State.ATTACK:
+		_on_attack()