netcode-revival: document Phase 1 status + two-instance test recipe

docs/netcode-revival.md

@@ -40,13 +40,31 @@ Implemented in `rexglue-sdk/src/kernel/xam/xam_net.cpp`:
 - `getsockopt`: real basic reads off the native socket.
 - `XNetSetSystemLinkPort`: accept.
 
-## Phase 1 — relay game-transport + session registry (next)
-- Extend `ghost_relay` to forward game UDP between room members + expose the
-  session list that `XNetQosLookup` reads.
-- Give rexglue `XSocket` a relay-transport mode (recognize `10.80.0.x`
-  virtual destinations; route via the relay outbound).
-- Test: two game instances on one machine (loopback through the relay) reach
-  a joined session.
+## Phase 1 — relay game-transport + session registry (done, built)
+- v5 relay: `kTypeGame` (opaque game UDP, unicast or room-broadcast) +
+  `kTypeSessionQuery`/`Reply` (discovery). JoinPayload carries the virtual IP.
+- rexglue `XNetTransport`: joins the same relay + room as the ghost client,
+  routes `sendto` to `10.80.0.x` peers through the relay, injects inbound
+  game datagrams into the game's sockets, and serves session discovery to
+  `XNetQosLookup`.
+- Wire structs byte-verified against the game's proto (sizes 7/31/1410/25/801)
+  with `static_assert`s on both sides.
+
+## Test recipe (two instances on one machine, local relay)
+1. Build: `cmake --build out/build/linux-relwithdebinfo --target ghost_relay skate3 -j$(nproc)`
+2. Start the local relay: `./out/build/linux-relwithdebinfo/ghost_relay --port 40001`
+3. Launch instance A: `./out/build/linux-relwithdebinfo/skate3`
+   Launch instance B (same machine; the relay loops them via 127.0.0.1).
+4. In each instance's M menu: set `skate3_ghost_server = 127.0.0.1`, enable
+   multiplayer, join the SAME room code. The relay then routes both the ghost
+   poses and the game's own netcode traffic.
+5. Verify in the log (stdout): `xnet transport: connecting to relay ...`,
+   `xnet transport: relay joined`, and on session search
+   `xnet transport: session reply with N session(s)`. Enable the game's
+   online flow (invite/join) and check whether RakNet establishes a session.
+6. Two instances bind the same UDP data port → the second may fail to bind.
+   If so, the game's socket bind must become relay-aware (bind to an
+   ephemeral port in relay mode) — a known Phase 3 item.
 
 ## Phase 2 — invite UI
 - `XamShowGameInviteUI` / `XamShowFriendsUI` as ImGui `XamDialog`s backed by