J3c: a land server stopping tells its players and is started afresh when they rejoin; the extra ports to forward on the Host page, the hosting message, the server log and HOSTING.md

docs/WORLDLETS_DESIGN.md

@@ -56,8 +56,17 @@ player list across lands are J3b). The rest is planned.
   asleep)" in chat.
 - `tests/test_net_lands.gd` covers it all, Alice in the Cove and Bob in the
   Valley.
-- Next, J3c: a land server crashing (its players back to the hub), and the
-  Host page's extra ports.
+
+*As built in J3c:*
+- A land server that stops (crashed, or killed) tells its players "Lost the
+  connection to the Cove. Join the server again to carry on there."; the
+  hub notices it's gone (even mid-join) and joining again starts it afresh
+  and sends them back (their characters were saved by its autosave).
+- The extra ports: `Lands.extra_ports()` / `ports_text()` ("UDP 24242 and
+  24243 (the Cove)"), on the Host page, in the "You're hosting" message, in
+  a hub's start-up log, and in docs/HOSTING.md.
+- Tested in `test_net_lands.gd` (the Cove's server killed, Alice rejoins)
+  and `test_worldlet_pick.gd` (the Host page).
 
 *As built in J1:*
 - `scripts/world/tunnel.gd`: the seam (an outcrop on the clifftop strip,