netcode-revival: document the revival plan + Phase 0 progress

docs/netcode-revival.md

@@ -0,0 +1,64 @@
+# Netcode revival (branch `netcode-revival`)
+
+Goal: revive Skate 3's **original** Xbox 360 multiplayer — the game's own
+RakNet-based peer-to-peer netcode — instead of only the ghost co-presence
+layer. `main` keeps the working ghost multiplayer + releases; this branch
+carries the netcode work and stays clearly separate.
+
+The transport is **RakNet over XNet/XSocket** (confirmed: rexglue's XNet
+comments reference RakNet, and the game's `socket → setsockopt → bind →
+sendto` sequence matches `RakPeer::Startup`). The game speaks its own
+protocol; we only need the **platform netcode layer to work** (implement
+rexglue's stubbed XNet session APIs + route the game's UDP through the
+relay). We do not decode the game protocol.
+
+## Architecture
+- **Virtual addressing**: each machine gets a stable `XNADDR` (virtual IP
+  `10.80.0.x` + unique 6-byte `abEnet` → RakNet GUID). `XNetXnAddrToInAddr` /
+  `XNetServerToInAddr` resolve those into the socket layer's address space.
+- **Relay = P2P stabilizer**: session UDP is forwarded outbound through the
+  relay (tagged `game` vs `ghost`) to the destination member's real socket;
+  on a LAN the transport can go direct. The server stabilizes NAT/remote
+  play but is not required on a LAN.
+- **Session registry in the relay**: `XNetQosListen` advertises a session;
+  `XNetQosLookup` returns discovered sessions; the lobby/room-code UI stays
+  as the manual join path.
+- **Ghost layer is kept** as the guaranteed fallback (co-presence that works
+  for everyone even if the netcode path fails).
+
+## Phase 0 — XNet session/address layer (done, rexglue `767bd6e`)
+Implemented in `rexglue-sdk/src/kernel/xam/xam_net.cpp`:
+- `XNetGetTitleXnAddr`: stable virtual identity (was loopback + `0xCC` MAC).
+- `XNetXnAddrToInAddr` / `XNetInAddrToXnAddr` / `XNetServerToInAddr` /
+  `XNetXnAddrToMachineId` / `XNetInAddrToString`: resolve virtual addresses
+  (were failure / `666.666.666.666`).
+- `XNetQosListen`: accept advertisement (was `X_ERROR_FUNCTION_FAILED`).
+- `XNetQosLookup`: success + empty `XNQOS` + completion signal (Phase 1
+  fills it from the relay registry).
+- `XNetCreateKey` / `XNetRegisterKey` / `XNetUnregisterKey`: session keys.
+- `XNetConnect` / `XNetGetConnectStatus`: report established.
+- `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 2 — invite UI
+- `XamShowGameInviteUI` / `XamShowFriendsUI` as ImGui `XamDialog`s backed by
+  the relay lobby (real player list). Receive side (`XamNotifyCreateListener`
+  / `XamSessionRefObjByHandle`) already works.
+
+## Phase 3 — iterate to playable
+Whatever the game needs past discovery (RakNet handshake, ports, privilege
+fakes). Ghost stays as fallback throughout.
+
+## Notes
+- Testing is done with **two instances on one machine** (no second device);
+  the relay loops them via `127.0.0.1`. A headless harness may be added.
+- Build: `clang-22` override, preset `linux-relwithdebinfo`, per
+  `docs/build.md` — same as `main`.