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]

scripts/ambience/GodraysOverlay.gd

@@ -0,0 +1,31 @@
+extends MeshInstance3D
+
+var godrays_material: ShaderMaterial
+
+func _ready() -> void:
+	var quad := QuadMesh.new()
+	quad.size = Vector2(20.0, 40.0)
+	mesh = quad
+
+	godrays_material = ShaderMaterial.new()
+	var shader := load("res://shaders/godrays.gdshader")
+	if shader:
+		godrays_material.shader = shader
+
+	var mat_override := godrays_material
+	material_override = mat_override
+
+	# Position in front of camera, slightly above center
+	position = Vector3(0.0, 5.0, -10.0)
+
+func _process(_delta: float) -> void:
+	if not godrays_material:
+		return
+
+	# Get player/camera world Y to drive fade_bottom
+	var cam := get_viewport().get_camera_3d()
+	if cam:
+		var world_y := cam.global_position.y
+		# Deeper = fade bottom rises, reducing godray reach
+		var dynamic_fade := clamp(world_y - 20.0, -100.0, 0.0)
+		godrays_material.set_shader_parameter("fade_bottom", dynamic_fade)

scripts/ambience/UnderwaterEnvironment.gd

@@ -16,13 +16,24 @@ func _ready() -> void:
 	env.fog_aerial_perspective = 0.3
 
 	env.volumetric_fog_enabled = true
-	env.volumetric_fog_density = 0.04
+	env.volumetric_fog_density = 0.05
 	env.volumetric_fog_albedo = Color(0.2, 0.5, 0.7)
 	env.volumetric_fog_emission = Color(0.02, 0.06, 0.1)
 	env.volumetric_fog_emission_energy = 0.1
 	env.volumetric_fog_length = 64.0
 	env.volumetric_fog_detail_spread = 2.0
 
+	# Attach custom fog shader
+	var fog_mat := FogMaterial.new()
+	var fog_shader := load("res://shaders/underwater_fog.gdshader")
+	if fog_shader:
+		fog_mat.set_shader(fog_shader)
+
+	var fog_volume := FogVolume.new()
+	fog_volume.size = Vector3(2000.0, 300.0, 2000.0)
+	fog_volume.material = fog_mat
+	add_child(fog_volume)
+
 	env.glow_enabled = true
 	env.glow_intensity = 0.5
 	env.glow_bloom = 0.1