Journey decisions: extendable ending, four crossings, biome hazards; server cost measured

docs/WORLDLETS_DESIGN.md

@@ -253,7 +253,34 @@ 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:** 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.
+
+### 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.
+
+### 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)?
 2. **The crossings:** do the four above fit (tunnel, sail, climb, burrow),