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/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