Expand link preview to cubic snapshot

docs/PORTING.md

@@ -138,6 +138,10 @@ window around the link point so the book retains a readable close-up.
 Version 0.1.19 renders the complete 9-by-64-by-9 snapshot in that Ponder
 scene while retaining the target-centered camera framing.
 
+Version 0.1.20 uses a cubic 18-by-18-by-18 snapshot. The renderer remaps the
+link point within that cube so its existing target-centered camera framing is
+unchanged.
+
 The preview code is separated behind a renderer interface and uses an isolated
 Ponder level populated from server-authoritative destination snapshots.