feat(world): open up the dive site fog and match the horizon colour

Lighter fog and volumetric fog, brighter key and ambient light, terrain
tiles visible to 420 m (60 m fade), and a background colour equal to
the fog colour times its energy so culled tiles never read as a dark
band. The seabed shader culls back faces again now that the tiles are
wound front-face up.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

game/world/sites/seabed_terrain.gdshader

@@ -1,5 +1,5 @@
 shader_type spatial;
-render_mode cull_disabled, depth_draw_opaque;
+render_mode depth_draw_opaque;
 
 uniform vec3 sand_color : source_color = vec3(0.50, 0.46, 0.34);
 uniform vec3 rock_color : source_color = vec3(0.21, 0.20, 0.18);