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/MULTIPLAYER_PLAN.md

@@ -17,6 +17,13 @@ concrete decisions for this codebase, as it stands after M1.
 - **Co-op or adversarial**, chosen by server rules (§5).
 
 ## 2. Running a server
+*Built in M2.1* (`scripts/net/net.gd`, `server_config.gd`, `identity.gd`).
+As of M2.1 the server accepts players, checks who they are and keeps the
+player list. The world itself isn't served yet (M2.3). Flags: `--server`,
+`--port`, `--config`, `--server-name`, `--max-players`, `--password`,
+`--world`, and `--exit-with <pid>` (quit when that process exits; the tests
+use it so a crashed test never leaves a server running). Flags apply to
+that run only and are never written to `server.json`.
 ```bash
 # Dedicated server (no window, no player)
 godot --headless --path . -- --server --config server.json
@@ -42,7 +49,9 @@ godot --headless --path . -- --server --world "Valley of Us" --port 24242
 
 ## 3. Players: identity and names
 - **Identity:** on first run each game install generates a private key
-  (Ed25519) and keeps it in `user://identity.key`. The public key's hash is
+  (RSA-2048, since Godot's Crypto has no Ed25519) and keeps it in
+  `user://identity/key.pem`, with the name and appearance in
+  `user://identity/profile.json`. The public key's hash is
   the **player id**. On joining, the client signs a server-chosen challenge,
   so nobody can claim someone else's id. No passwords and no central
   accounts.
@@ -199,7 +208,7 @@ player" or "this player":
 | Phase | Delivers | Done when |
 |---|---|---|
 | **M2.0** | worldlet W0 (terrain from data, no visible change), recommended first | seed 1337 generates the same height grid as today; all suites pass |
-| **M2.1** | network skeleton: `--server`, ENet on 24242, Join screen (address, name), handshake with identity and challenge, name rules, player list | two clients and a headless server on one Mac connect, see the player list, and disconnect cleanly |
+| **M2.1** *(done)* | network skeleton: `--server`, ENet on 24242, Join screen (address, name), handshake with identity and challenge, name rules, player list | two clients and a headless server on one Mac connect, see the player list, and disconnect cleanly |
 | **M2.2** | player bodies and name tags; predicted movement, interpolation for others | two players walk around together at 100 ms simulated latency without visible rubber-banding |
 | **M2.3** | world state replicated: resource nodes, pickups, pieces, chests, fires, doors, clock, weather | one chops a tree and the other sees it fall; one builds a wall and the other walks into it |
 | **M2.4** | intents as RPCs; per-player inventory, stats, diet, death and graves; per-player saves and migration | two players craft, eat, die and loot their own graves; an old single-player save loads |

docs/ROADMAP.md

@@ -44,7 +44,7 @@ Detailed plan in `docs/MULTIPLAYER_PLAN.md` (hosting on UDP 24242 via Tailscale
 or port forwarding, player identity and names, server rules for co-op/PvP, and
 phases M2.0-M2.7).
 - [ ] Refactor state to server-authoritative (single-player becomes loopback server)
-- [ ] Headless dedicated server mode (`--headless -- --server`)
+- [x] Headless dedicated server, joining, identity handshake, names, player list *(M2.1)*
 - [ ] ENet transport, client prediction for local movement, interpolation for others
 - [ ] Replicate resource nodes, inventory, building, health
 - [ ] Persistence: world save/load on the server

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
 ```