Journey: one boat each, no passengers; tunnels, ropes and burrows shared

docs/WORLDLETS_DESIGN.md

@@ -275,6 +275,16 @@ A server today runs one island. Options:
     place, load or wake the other worldlet, send them its world);
   - sleep: "everyone in bed" counts the players in that worldlet only;
   - the Old Stag and other bosses scale with the players in their worldlet.
+- **Boats: one each, no passengers.** A boat belongs to whoever built it
+  (like a bed): only the owner, and their friends under the server's locks
+  rule, can sail it, and nobody else can dismantle it. It stays moored where
+  it was last left (the Cove end or the Highlands end), so nobody is
+  stranded by someone else taking the only boat. Lose or wreck it and you
+  build another, which is harder where wood is scarce (the Highlands), so
+  plan the trip back. Each player has to earn their own crossing by boat.
+- **The other crossings are shared:** a dug tunnel, a fixed climbing rope
+  and a dug burrow are changes to the world, there for everyone once one
+  player has made them (like a door or a bridge).
 
 ### Server cost, measured (0.3.1)
 One worldlet, a dedicated server on the M5 Pro, never pausing: about