feat(world): depth-attenuated daylight and cold-water palette for the seabed

The seabed shader applies per-channel Beer-Lambert transmittance relative
to a 24 m reference depth: the scalar part dims the directional light in a
custom light() function and the sky ambient through AO, the chroma part
tints the albedo, and artificial lights are left untouched for the
submarine's projectors. Sky, fog and sun are cooled and the sun softened
so the plateau reads green-blue, the canyon floor cold blue and the
drop-off foot near-indigo.

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

game/world/sites/dive_site.tscn

@@ -4,12 +4,12 @@
 [ext_resource type="Script" path="res://game/world/sites/bathymetric_terrain_actor.gd" id="2_terrain"]
 
 [sub_resource type="ProceduralSkyMaterial" id="SkyUnderwater"]
-sky_top_color = Color(0.3, 0.62, 0.64, 1)
-sky_horizon_color = Color(0.13, 0.36, 0.41, 1)
+sky_top_color = Color(0.22, 0.52, 0.66, 1)
+sky_horizon_color = Color(0.09, 0.28, 0.4, 1)
 sky_curve = 0.2
 sky_energy_multiplier = 1.0
-ground_horizon_color = Color(0.11, 0.3, 0.35, 1)
-ground_bottom_color = Color(0.015, 0.05, 0.07, 1)
+ground_horizon_color = Color(0.05, 0.17, 0.27, 1)
+ground_bottom_color = Color(0.01, 0.03, 0.07, 1)
 ground_curve = 0.1
 ground_energy_multiplier = 0.6
 sun_angle_max = 40.0
@@ -26,7 +26,7 @@ background_mode = 2
 sky = SubResource("SkyResource")
 ambient_light_source = 3
 ambient_light_sky_contribution = 1.0
-ambient_light_energy = 1.0
+ambient_light_energy = 1.3
 reflected_light_source = 1
 tonemap_mode = 2
 tonemap_exposure = 1.15
@@ -48,8 +48,8 @@ glow_intensity = 0.8
 glow_blend_mode = 0
 glow_hdr_threshold = 1.2
 fog_enabled = true
-fog_light_color = Color(0.1, 0.3, 0.35, 1)
-fog_light_energy = 0.52
+fog_light_color = Color(0.05, 0.18, 0.29, 1)
+fog_light_energy = 0.38
 fog_sun_scatter = 0.15
 fog_density = 0.006
 fog_aerial_perspective = 0.5
@@ -57,7 +57,7 @@ fog_height = -4.0
 fog_height_density = 0.025
 volumetric_fog_enabled = true
 volumetric_fog_density = 0.01
-volumetric_fog_albedo = Color(0.6, 0.85, 0.9, 1)
+volumetric_fog_albedo = Color(0.5, 0.78, 0.92, 1)
 volumetric_fog_anisotropy = 0.45
 volumetric_fog_length = 72.0
 volumetric_fog_ambient_inject = 0.5
@@ -73,11 +73,11 @@ environment = SubResource("EnvironmentUnderwater")
 
 [node name="AmbientDirection" type="DirectionalLight3D" parent="."]
 rotation_degrees = Vector3(-62, -28, 0)
-light_color = Color(0.56, 0.8, 0.82, 1)
-light_energy = 1.35
+light_color = Color(0.5, 0.78, 0.88, 1)
+light_energy = 0.9
 light_angular_distance = 3.0
 shadow_enabled = true
-shadow_opacity = 0.7
+shadow_opacity = 0.45
 shadow_blur = 2.0
 directional_shadow_max_distance = 160.0
 directional_shadow_fade_start = 0.75