Implement grid biome distribution

docs/PORTING.md

@@ -335,6 +335,11 @@ lighting, weather, day length, celestial angle, and celestial phase on the
 book's right-hand description page. This works for held books and Book Stands
 without copying mutable Age-generation data onto the item.
 
+Version 0.2.23.0 applies Grid Biome Distribution using two deterministic
+Age-seeded biomes in Legacy's diagonal checkerboard layout. Each cell is one
+16-by-16-block chunk, and the built-in serializable biome source reconstructs
+the same layout whenever the Age reloads.
+
 Version 0.2.14.0 restores the legacy Linking and Descriptive Book artwork for
 inventory items and gives each book its matching open-book texture when placed
 on a Book Stand. Return Linking Books created inside another Age also retain and