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>

shaders/water_surface.gdshader

@@ -0,0 +1,21 @@
+shader_type spatial;
+render_mode cull_front, depth_draw_never, blend_mix;
+
+uniform vec3 surface_color : source_color = vec3(0.15, 0.45, 0.65);
+uniform float wave_speed = 0.3;
+uniform float wave_amplitude = 0.15;
+
+void vertex() {
+	float wave1 = sin(VERTEX.x * 0.5 + TIME * wave_speed) * wave_amplitude;
+	float wave2 = cos(VERTEX.z * 0.4 + TIME * wave_speed * 1.3) * wave_amplitude * 0.7;
+	VERTEX.y += wave1 + wave2;
+}
+
+void fragment() {
+	ALBEDO = surface_color;
+	float fresnel = pow(1.0 - dot(NORMAL, VIEW), 2.0);
+	ALPHA = 0.4 + fresnel * 0.4;
+	METALLIC = 0.2;
+	ROUGHNESS = 0.3;
+	EMISSION = surface_color * 0.1;
+}