Apply single biome Age distribution

docs/PORTING.md

@@ -309,6 +309,14 @@ instead of advancing and correcting their time once per tick. This keeps the
 integer day time fixed while preventing the client skybox from visually
 bouncing between adjacent celestial positions.
 
+Version 0.2.21.0 begins applying persisted biome-distribution traits. Native
+Distribution retains Minecraft's data-pack-aware Overworld climate source.
+Single Biome Distribution deterministically selects one registered biome from
+the Age seed and uses it throughout compatible noise-based terrain. Flat and
+Void profiles retain their profile-specific biome sources. Legacy's scaled,
+tiled, and grid layouts require dedicated modern biome sources and remain the
+next distribution work.
+
 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