Journey: multiplayer spread out (decided)

docs/WORLDLETS_DESIGN.md

@@ -263,9 +263,18 @@ A server today runs one island. Options:
   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:** leaning to *spread out* (each player goes where they
-  like), if a server can run several worldlets at once. A measurement of
-  today's server, in the next section, says whether it can.
+- **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.
 
 ### Server cost, measured (0.3.1)
 One worldlet, a dedicated server on the M5 Pro, never pausing: about
@@ -276,10 +285,6 @@ 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.
 
-### Questions still open
-1. **Multiplayer:** spread out (confirm, now the cost is known), or everyone
-   together after all?
-
 ### (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)?