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/caustics.gdshader

@@ -1,21 +1,33 @@
 shader_type spatial;
-render_mode unshaded, blend_add, cull_disabled, depth_draw_never;
+render_mode unshaded, blend_add, depth_draw_never, cull_disabled;
 
-uniform float speed : hint_range(0.1, 3.0) = 0.8;
-uniform float scale : hint_range(1.0, 20.0) = 8.0;
-uniform float intensity : hint_range(0.0, 2.0) = 0.6;
-uniform vec3 caustic_color : source_color = vec3(0.7, 0.9, 1.0);
+uniform vec3 caustic_color : source_color = vec3(0.5, 0.8, 1.0);
+uniform float intensity : hint_range(0, 3) = 1.5;
+uniform float scale = 8.0;
+uniform float speed = 0.5;
+
+float caustic_pattern(vec2 uv) {
+	float t = TIME * speed;
+	vec2 p = uv * scale;
+	float c = 0.0;
+	c += sin(p.x + t) * 0.5;
+	c += cos(p.y + t * 0.7) * 0.5;
+	c += sin(p.x * 0.7 + p.y * 0.5 + t * 1.3) * 0.3;
+	c = abs(c);
+	c = smoothstep(0.2, 0.0, c);
+	return c;
+}
 
 void fragment() {
-	vec2 uv = UV * scale;
+	vec2 uv = UV * 2.0 - 1.0;
+	float c1 = caustic_pattern(uv);
+	float c2 = caustic_pattern(uv * 1.3 + vec2(0.5, 0.3));
+	float caustics = (c1 + c2 * 0.5) * intensity;
 
-	float pattern1 = sin(TIME * speed + uv.x * 10.0 + sin(uv.y * 10.0 + TIME * speed));
-	float pattern2 = sin(TIME * speed * 0.7 + uv.y * 12.0 + sin(uv.x * 8.0 + TIME * speed * 1.3));
-	float pattern3 = sin(TIME * speed * 1.1 + (uv.x + uv.y) * 7.0 + sin(uv.x * 5.0 - TIME * speed * 0.5));
+	// Fade avec profondeur
+	float depth_fade = clamp((VERTEX.y + 60.0) / 80.0, 0.0, 1.0);
+	caustics *= depth_fade;
 
-	float caustic = (pattern1 + pattern2 + pattern3) / 3.0;
-	caustic = pow(max(caustic, 0.0), 2.0) * intensity;
-
-	EMISSION = caustic_color * caustic;
-	ALBEDO = vec3(0.0);
+	ALBEDO = caustic_color * caustics;
+	ALPHA = caustics;
 }