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

project.godot

@@ -25,6 +25,7 @@ config/icon="res://icon.svg"
 ItemDB="*res://scripts/core/item_db.gd"
 Actions="*res://scripts/core/actions.gd"
 Saves="*res://scripts/core/saves.gd"
+Net="*res://scripts/net/net.gd"
 
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