M2.1: dedicated server, joining, identity handshake, names, lobby

- Net autoload: headless dedicated server (--server) on UDP 24242 via ENet,
  server.json preferences (flags override per run, never saved), reserved
  names in names.json
- Identity: RSA-2048 key per install (Godot has no Ed25519), player id =
  key hash, challenge/response so ids can't be faked; profile with name
  and limited appearance (tunic, hair, beard)
- Server refuses bans, allowlist misses, duplicate logins, bad or taken
  names, wrong passwords, old protocol versions and a full server, each
  with a clear reason; first player on a new server becomes admin
- Title screen: Profile, Join Game, and a lobby with server info, rules
  and who's online
- test_net_lobby: real headless server + bot processes over UDP (38
  checks); --exit-with so test servers never outlive the test

docs/SETUP_ON_NEW_MACHINE.md

@@ -36,6 +36,8 @@ godot --headless --path . --script res://tests/test_terrain_edges.gd
 godot --headless --path . --script res://tests/test_survival_core.gd
 # Save/load round trip + world slots (uses user://test_worlds, not your saves)
 godot --headless --path . --script res://tests/test_save_load.gd
+# Multiplayer lobby: a real headless server on UDP 24343 + scripted bot players
+godot --headless --path . --script res://tests/test_net_lobby.gd
 # Survival stats: hunger, warmth, fire fuel, cooking, foraging, death + grave
 godot --headless --path . --script res://tests/test_survival_stats.gd
 ```