feat(world): triplanar seabed shader with procedural normal maps and detail shading

Rock is normal-mapped triplanar in world space at two scales, sand carries an
oriented ripple normal map on the xz plane, and a grain map drives detail
occlusion, albedo mottling and roughness. The three maps are seamless,
seeded, mipmapped textures built synchronously by SeabedDetailTextures, so
no asset is needed and builds are deterministic. The mesh has no tangents:
perturbed normals are computed in world space and written to NORMAL.

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

game/world/sites/bathymetric_terrain_actor.gd

@@ -2,14 +2,17 @@ class_name BathymetricTerrainActor
 extends StaticBody3D
 
 const TerrainHeightField = preload("res://game/domain/sites/terrain_height_field.gd")
+const SeabedDetailTextures = preload("res://game/world/sites/seabed_detail_textures.gd")
 const TERRAIN_SHADER = preload("res://game/world/sites/seabed_terrain.gdshader")
 
 const TILE_CELLS := 64
 const VISIBILITY_RANGE_END_M := 420.0
 const VISIBILITY_RANGE_MARGIN_M := 60.0
+const DETAIL_TEXTURE_SEED := 4102
 
 var _field: TerrainHeightField
 var _material: ShaderMaterial
+var _detail_textures: SeabedDetailTextures
 var _tiles_root: Node3D
 var _collision: CollisionShape3D
 
@@ -19,8 +22,13 @@ func configure(field: TerrainHeightField) -> String:
 		return "terrain field is missing"
 	_clear()
 	_field = field
+	if _detail_textures == null:
+		_detail_textures = SeabedDetailTextures.build(DETAIL_TEXTURE_SEED)
 	_material = ShaderMaterial.new()
 	_material.shader = TERRAIN_SHADER
+	_material.set_shader_parameter(&"sand_normal_map", _detail_textures.sand_normal)
+	_material.set_shader_parameter(&"rock_normal_map", _detail_textures.rock_normal)
+	_material.set_shader_parameter(&"rock_grain_map", _detail_textures.rock_grain)
 	_build_tiles()
 	_build_collision()
 	return ""
@@ -30,6 +38,10 @@ func height_field() -> TerrainHeightField:
 	return _field
 
 
+func terrain_material() -> ShaderMaterial:
+	return _material
+
+
 func tile_count() -> int:
 	return 0 if _tiles_root == null else _tiles_root.get_child_count()