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

tests/net_bot.gd

@@ -0,0 +1,57 @@
+extends SceneTree
+## A scripted headless player for the network tests (tests/test_net_lobby.gd
+## starts several of these as separate processes).
+##
+## Args (after --): --identity <dir> --name <name> --address host:port
+##   [--password p] [--stay seconds] [--result <json path>] [--forge] [--protocol n]
+## Joins, stays a while noting who it saw, leaves, writes the outcome to the
+## result file and quits. Gives up after 20 s whatever happens.
+
+var _outcome := ""
+var _reason := ""
+
+func _initialize() -> void:
+	_run.call_deferred()
+
+func _run() -> void:
+	var net: Node = root.get_node("Net")
+	var args: Dictionary = net.parse_args(OS.get_cmdline_user_args())
+	var Identity = load("res://scripts/net/identity.gd")
+	var me = Identity.open(String(args.get("identity", "user://test_net/bot")))
+	if args.has("name"):
+		me.name = String(args.name)
+	net.identity = me
+	net.debug_forge = args.has("forge")
+	net.debug_protocol = int(args.get("protocol", 0))
+	net.debug_skip_name_check = args.has("raw-name")
+	net.joined.connect(func(): _outcome = "joined")
+	net.join_failed.connect(func(r): _outcome = "rejected"; _reason = r)
+	var res := {"joined": false, "reason": "", "saw": [], "admin": false, "server_name": "", "id": me.player_id}
+	net.join(String(args.get("address", "127.0.0.1")), String(args.get("password", "")))
+	var start := Time.get_ticks_msec()
+	while _outcome.is_empty() and Time.get_ticks_msec() - start < 20000:
+		await process_frame
+	if _outcome == "joined":
+		res.joined = true
+		res.server_name = net.server_info.get("server_name", "")
+		res.admin = bool(net.players.get(net.local_peer, {}).get("admin", false))
+		var saw := {}
+		var stay := float(args.get("stay", 2.0))
+		var s := 0.0
+		while s < stay and net.state == "connected":
+			for p in net.players.values():
+				saw[p.name] = true
+			await create_timer(0.1).timeout
+			s += 0.1
+		res.saw = saw.keys()
+		net.leave()
+		for _i in range(10):
+			await process_frame
+	else:
+		res.reason = _reason if not _reason.is_empty() else "timed out"
+	if args.has("result"):
+		var f := FileAccess.open(String(args.result), FileAccess.WRITE)
+		f.store_string(JSON.stringify(res))
+		f.close()
+	print("[bot %s] %s" % [me.name, JSON.stringify(res)])
+	quit()