Require footing for descriptive book travel

docs/PORTING.md

@@ -156,6 +156,9 @@ Version 0.2.2 adds the same server-provided Ponder destination preview to
 Descriptive Books. If no snapshot is available, the left page shows an
 animated linking-panel fallback.
 
+Version 0.2.3 requires Descriptive Book destinations to have a sturdy floor,
+while Linking Books continue to permit passable floorless destinations.
+
 The preview code is separated behind a renderer interface and uses an isolated
 Ponder level populated from server-authoritative destination snapshots.