feat(world): world-space marine snow around the viewer

A GPUParticles3D emitter in the dive site re-centres on the active camera
every frame and restarts after a jump, so flakes keep their own slow sink
and drift while the submarine moves through them. Soft additive billboard
discs dimmed by the water column and faded over their life replace the
sphere volume that was attached to the submarine and turned with it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

game/world/sites/dive_site.tscn

@@ -1,9 +1,10 @@
-[gd_scene load_steps=10 format=3]
+[gd_scene load_steps=11 format=3]
 
 [ext_resource type="Script" path="res://game/world/sites/dive_site.gd" id="1_site"]
 [ext_resource type="Script" path="res://game/world/sites/bathymetric_terrain_actor.gd" id="2_terrain"]
 [ext_resource type="Shader" path="res://game/world/sites/water_column_fog.gdshader" id="3_water_column"]
 [ext_resource type="Script" path="res://game/world/sites/seabed_flora_actor.gd" id="4_flora"]
+[ext_resource type="Script" path="res://game/world/sites/marine_snow.gd" id="5_snow"]
 
 [sub_resource type="ShaderMaterial" id="WaterColumnMaterial"]
 shader = ExtResource("3_water_column")
@@ -101,5 +102,8 @@ script = ExtResource("2_terrain")
 unique_name_in_owner = true
 script = ExtResource("4_flora")
 
+[node name="MarineSnow" type="GPUParticles3D" parent="."]
+script = ExtResource("5_snow")
+
 [node name="Placements" type="Node3D" parent="."]
 unique_name_in_owner = true