feat(world): underwater sky ambient, soft shadows and SSAO for the dive site

The environment declared the sky as its ambient source without a sky, so
shadowed faces received no ambient light. A procedural underwater sky now
feeds ambient and reflections; the directional light is steeper and
stronger with 70 % soft shadows; SSAO, filmic tonemapping, aerial
perspective and tinted volumetric fog complete the pass.

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

docs/briefs/terrain-site-zone1-gameplay.md

@@ -250,6 +250,13 @@ de contacts existant les sépare du fond.
   du tombant, têtes de corail sur les patch reefs, rien d'animé.
 - Brouillard, lumière directionnelle et particules de la scène de test
   actuelle sont conservés et réglés par site.
+- Passe lumière du 2026-10-06 (`dive_site.tscn`) : l'ambiance vient d'un ciel
+  procédural sous-marin (haut turquoise, horizon aligné sur le brouillard, bas
+  presque noir), ce qui éclaire les faces à l'ombre ; la source d'ambiance
+  « ciel » était déclarée sans ciel, d'où les ombres noires. Directionnelle
+  plus haute et plus forte (62°, 1,35) avec ombres douces à 70 % d'opacité,
+  SSAO pour l'occlusion à l'échelle du maillage, tonemap filmique, perspective
+  aérienne et albédo du brouillard volumétrique teintés.
 
 **État du 2026-10-06 (demande d'Olivier)** : le shader mélange sable et roche
 par la pente à partir de 20° (sable sous 20°, roche au-dessus de 40°, fondu

game/world/sites/dive_site.tscn

@@ -1,31 +1,69 @@
-[gd_scene load_steps=4 format=3]
+[gd_scene load_steps=6 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"]
 
+[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_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_curve = 0.1
+ground_energy_multiplier = 0.6
+sun_angle_max = 40.0
+sun_curve = 0.3
+use_debanding = true
+
+[sub_resource type="Sky" id="SkyResource"]
+sky_material = SubResource("SkyUnderwater")
+process_mode = 2
+radiance_size = 2
+
 [sub_resource type="Environment" id="EnvironmentUnderwater"]
-background_mode = 1
-background_color = Color(0.034, 0.12, 0.144, 1)
+background_mode = 2
+sky = SubResource("SkyResource")
 ambient_light_source = 3
-ambient_light_color = Color(0.12, 0.32, 0.37, 1)
-ambient_light_energy = 0.55
+ambient_light_sky_contribution = 1.0
+ambient_light_energy = 1.0
+reflected_light_source = 1
+tonemap_mode = 2
+tonemap_exposure = 1.15
+tonemap_white = 4.0
+ssao_enabled = true
+ssao_radius = 2.5
+ssao_intensity = 2.2
+ssao_power = 1.6
+ssao_detail = 0.6
+ssao_horizon = 0.08
+ssao_light_affect = 0.25
+ssao_ao_channel_affect = 0.4
 glow_enabled = true
 glow_levels/1 = 1.0
 glow_levels/2 = 1.0
 glow_levels/3 = 0.6
 glow_levels/4 = 0.2
-glow_intensity = 1.0
+glow_intensity = 0.8
 glow_blend_mode = 0
-glow_hdr_threshold = 1.0
+glow_hdr_threshold = 1.2
 fog_enabled = true
-fog_light_color = Color(0.07, 0.25, 0.3, 1)
-fog_light_energy = 0.48
+fog_light_color = Color(0.1, 0.3, 0.35, 1)
+fog_light_energy = 0.52
+fog_sun_scatter = 0.15
 fog_density = 0.006
+fog_aerial_perspective = 0.5
 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_anisotropy = 0.45
 volumetric_fog_length = 72.0
+volumetric_fog_ambient_inject = 0.5
+adjustment_enabled = true
+adjustment_contrast = 1.06
+adjustment_saturation = 1.08
 
 [node name="DiveSite" type="Node3D"]
 script = ExtResource("1_site")
@@ -34,10 +72,15 @@ script = ExtResource("1_site")
 environment = SubResource("EnvironmentUnderwater")
 
 [node name="AmbientDirection" type="DirectionalLight3D" parent="."]
-rotation_degrees = Vector3(-52, -24, 0)
-light_color = Color(0.39, 0.65, 0.7, 1)
-light_energy = 0.75
+rotation_degrees = Vector3(-62, -28, 0)
+light_color = Color(0.56, 0.8, 0.82, 1)
+light_energy = 1.35
+light_angular_distance = 3.0
 shadow_enabled = true
+shadow_opacity = 0.7
+shadow_blur = 2.0
+directional_shadow_max_distance = 160.0
+directional_shadow_fade_start = 0.75
 
 [node name="Terrain" type="StaticBody3D" parent="."]
 unique_name_in_owner = true