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/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)