feat(particles): bubble trail behind dolphin + block break burst

- BubbleTrail.gd: GPUParticles3D, auto-configured in _ready, set_intensity() API
- BlockBreakParticles.gd: one_shot burst, emit_burst(pos, color) API
- DolphinController.gd: bubble_trail onready + speed_factor hook in _update_movement
- Dolphin.tscn: BubbleEmitterPoint (0,0,1.2) > BubbleTrail child added

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

scripts/ambience/CausticsLayer.gd

@@ -0,0 +1,47 @@
+extends Node3D
+
+# Creates horizontal caustic planes at multiple depths, follows the player
+# Add as child of any node that follows the player (e.g. PlanktonFollower)
+
+const LAYER_DEPTHS := [-5.0, -15.0, -25.0]
+const LAYER_SIZE := 60.0
+
+var caustic_meshes: Array[MeshInstance3D] = []
+var caustic_material: ShaderMaterial
+
+func _ready() -> void:
+	var shader := load("res://shaders/caustics.gdshader")
+	if not shader:
+		push_warning("CausticsLayer: caustics.gdshader not found")
+		return
+
+	caustic_material = ShaderMaterial.new()
+	caustic_material.shader = shader
+
+	for depth in LAYER_DEPTHS:
+		var mi := MeshInstance3D.new()
+		var plane := PlaneMesh.new()
+		plane.size = Vector2(LAYER_SIZE, LAYER_SIZE)
+		mi.mesh = plane
+		# Each layer gets its own material instance to allow per-depth tuning
+		var mat := caustic_material.duplicate() as ShaderMaterial
+		# Lower layers = less intense
+		var fade := clamp(1.0 + depth / 30.0, 0.0, 1.0)
+		mat.set_shader_parameter("intensity", 1.5 * fade)
+		mi.material_override = mat
+		mi.position = Vector3(0.0, depth, 0.0)
+		add_child(mi)
+		caustic_meshes.append(mi)
+
+func _process(_delta: float) -> void:
+	# Follow camera XZ, keep fixed Y depths
+	var cam := get_viewport().get_camera_3d()
+	if cam:
+		var cx := cam.global_position.x
+		var cz := cam.global_position.z
+		for i in caustic_meshes.size():
+			var mi := caustic_meshes[i]
+			mi.global_position.x = cx
+			mi.global_position.z = cz
+			# Keep the fixed depth offset
+			mi.global_position.y = LAYER_DEPTHS[i]