Scan Age spawn columns from world top

docs/PORTING.md

@@ -202,6 +202,10 @@ no suitable natural footing exists.
 Version 0.2.10.2 allows leaf blocks to count as proper natural footing when
 selecting the highest arrival surface.
 
+Version 0.2.10.3 scans each candidate column downward from the dimension's top
+before choosing footing, preventing cave floors from being selected as the Age
+arrival surface. Columns covered by fluid remain ineligible.
+
 The preview code is separated behind a renderer interface and uses an isolated
 Ponder level populated from server-authoritative destination snapshots.