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

docs/WORLDLETS_DESIGN.md

@@ -253,7 +253,49 @@ A server today runs one island. Options:
   The most freedom, and the most work and CPU.
 - **Party leader:** whoever goes through brings the others with a vote.
 
-### Questions for you
+### Decisions (Paul, Nov 2026)
+- **Extendable, not finished:** the Ember has a last boss, and after it you
+  keep playing. The chain is built so more worldlets (and crossings) can be
+  added after the Ember later: each worldlet names the next, nothing assumes
+  the Ember is the end.
+- **The crossings:** tunnel, sail, climb, burrow, in that order.
+- **Harder by biome, not just by numbers:** each worldlet has its own,
+  stronger animals and its own hazards. The Marsh, for one: mosquitoes whose
+  bites can make you sick, and bog, mist and the like. Others in the same
+  spirit per biome (to design with each worldlet).
+- **Multiplayer: spread out.** Each player goes where they like; the server
+  runs every worldlet someone is in, and the rest stand still. Built in J3,
+  after single-player travel works. What that means for the code:
+  - one server process holds several worldlet scenes, each with its own
+    clock, weather, creatures and save file;
+  - every player belongs to one worldlet at a time, and the server only
+    sends them that worldlet's world, creatures, sounds and chat-free events
+    (chat and the player list stay server-wide);
+  - a crossing moves a player from one worldlet to another (save their
+    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
+**12% of one CPU core and 270 MB of memory**, with nobody on. Players add
+creatures, weather and pickups around them. So with *spread out*, the worst
+case of 8 players in all 5 worldlets is about five of today's servers in
+one process: roughly 1.4 GB and a little over half a core before players,
+and worldlets with nobody in them stand still and cost next to nothing.
+Comfortable on the home Mac; a small cloud server (2 GB) would be tight.
+
+### (Answered) questions
 1. **The ending:** a real ending at the Ember (light the beacon, a last
    boss, the credits) or open-ended (the journey is the goal, keep living)?
 2. **The crossings:** do the four above fit (tunnel, sail, climb, burrow),