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>

tests/integration/run_site_terrain_integration.gd

@@ -20,7 +20,7 @@ const DIVE_SITE_SCENE = preload("res://game/world/sites/dive_site.tscn")
 const ZONE1_SITE_PATH := "res://content/sites/zone1_vigilant.tres"
 const SHOWCASE_SITE_PATH := "res://content/sites/terrain_showcase.tres"
 const GROOVE_PROBE_OFFSET_M := 25.0
-const TEST_COUNT := 51
+const TEST_COUNT := 52
 
 var _failures: Array[String] = []
 
@@ -210,6 +210,8 @@ func _test_dive_site_scene() -> void:
 	var first_pass := flora.sample_transforms(SeabedFloraActor.Kind.SEAGRASS)
 	flora.configure(field_for_flora, hash(String(definition.site_id)))
 	_expect(first_pass == flora.sample_transforms(SeabedFloraActor.Kind.SEAGRASS), "flora placement is deterministic for a seed")
+	var snow := site.get_node_or_null("MarineSnow") as GPUParticles3D
+	_expect(snow != null and snow.emitting and snow.amount > 0 and not snow.local_coords, "site carries world-space marine snow around the viewer")
 	var water_column := site.get_node_or_null("WaterColumn") as FogVolume
 	_expect(
 		water_column != null and water_column.material is ShaderMaterial