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/marine_snow.gdshader

@@ -0,0 +1,36 @@
+shader_type spatial;
+render_mode blend_add, depth_draw_never, cull_disabled, unshaded;
+
+// Marine snow: camera-facing soft discs drawn by GPUParticles3D. Each particle is a
+// unit quad; the flake is a radial falloff in UV. Brightness follows the daylight of
+// the water column (so flakes dim with depth) and fades in and out over the life of
+// the particle through COLOR.a set by the process material.
+
+#include "res://game/world/sites/water_light.gdshaderinc"
+
+uniform vec3 flake_color : source_color = vec3(0.78, 0.86, 0.9);
+uniform float flake_strength = 1.4;
+uniform vec3 water_absorption_per_m = vec3(0.11, 0.04, 0.032);
+uniform float reference_depth_m = 24.0;
+uniform float minimum_daylight = 0.08;
+
+varying float brightness;
+
+void vertex() {
+	// Billboard: keep the particle's world position, face the camera.
+	vec3 world_origin = (MODEL_MATRIX * vec4(0.0, 0.0, 0.0, 1.0)).xyz;
+	float size = length(MODEL_MATRIX[0].xyz);
+	mat3 view_basis = mat3(INV_VIEW_MATRIX);
+	vec3 world = world_origin + (view_basis[0] * VERTEX.x + view_basis[1] * VERTEX.y) * size;
+	VERTEX = (VIEW_MATRIX * vec4(world, 1.0)).xyz;
+	NORMAL = (VIEW_MATRIX * vec4(view_basis[2], 0.0)).xyz;
+	vec3 transmittance = water_transmittance(-world_origin.y, water_absorption_per_m, reference_depth_m);
+	brightness = water_daylight(transmittance, minimum_daylight) * COLOR.a;
+}
+
+void fragment() {
+	float radial = length(UV - vec2(0.5)) * 2.0;
+	float disc = 1.0 - smoothstep(0.35, 1.0, radial);
+	ALBEDO = flake_color * disc * brightness * flake_strength;
+	ALPHA = disc;
+}