feat(world): volumetric coral heads on patch reefs

Three procedural coral head meshes (boulder, brain-ridged, lobed domes on
a flat base, vertex-shaded crevices) are scattered on gentle rock between
12 and 40 m, clustered by noise, one MultiMesh per tile and variant, with
an opaque shader under the shared water column. The coral assertions
run on the Banc du Vigilant, whose patch reef lies in the lit band.

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

docs/briefs/captures/zone1_vigilant_patch_reef.png.import

@@ -0,0 +1,40 @@
+[remap]
+
+importer="texture"
+type="CompressedTexture2D"
+uid="uid://bfhdetospcaup"
+path="res://.godot/imported/zone1_vigilant_patch_reef.png-c13e4b56c466e2ab61173a145fd844d2.ctex"
+metadata={
+"vram_texture": false
+}
+
+[deps]
+
+source_file="res://docs/briefs/captures/zone1_vigilant_patch_reef.png"
+dest_files=["res://.godot/imported/zone1_vigilant_patch_reef.png-c13e4b56c466e2ab61173a145fd844d2.ctex"]
+
+[params]
+
+compress/mode=0
+compress/high_quality=false
+compress/lossy_quality=0.7
+compress/uastc_level=0
+compress/rdo_quality_loss=0.0
+compress/hdr_compression=1
+compress/normal_map=0
+compress/channel_pack=0
+mipmaps/generate=false
+mipmaps/limit=-1
+roughness/mode=0
+roughness/src_normal=""
+process/channel_remap/red=0
+process/channel_remap/green=1
+process/channel_remap/blue=2
+process/channel_remap/alpha=3
+process/fix_alpha_border=true
+process/premult_alpha=false
+process/normal_map_invert_y=false
+process/hdr_as_srgb=false
+process/hdr_clamp_exposure=false
+process/size_limit=0
+detect_3d/compress_to=1