Lock preview view to local scene

docs/PORTING.md

@@ -131,6 +131,10 @@ Version 0.1.17 changes destination previews to a 9-by-128-by-9 volume: a
 square horizontal footprint with a vertical slice spanning 64 blocks below and
 63 blocks above the recorded linking location.
 
+Version 0.1.18 changes the preview cache to 9-by-64-by-9, spanning 32 blocks
+below and 31 above the link point. The Ponder section remains locked to the
+centered 9-by-7-by-9 window so the book retains a readable close-up.
+
 The preview code is separated behind a renderer interface and uses an isolated
 Ponder level populated from server-authoritative destination snapshots.