add capnproto schema

world/README.md

@@ -18,5 +18,4 @@ This DWN acts as a location for world discovery.
 
 ## Networking
 
-Networking within an instance uses [WebTransport](https://developer.mozilla.org/en-US/docs/Web/API/WebTransport),
-following a [protobuf](https://protobuf.dev/) specification.
+Networking within an instance follows a [Cap'n Proto](https://capnproto.org/) schema over [WebTransport](https://developer.mozilla.org/en-US/docs/Web/API/WebTransport).

world/schemas/world_server.capnp

@@ -0,0 +1,27 @@
+@0xae09eca2c525e50a;
+
+struct Result(T) {
+  union {
+    success @0: T;
+    error @1: Text;
+  }
+}
+
+interface WorldServer {
+  joinInstance @0 (instance: Instance) -> (response :Result(JoinResponse));
+
+  struct Instance {
+    recordId @0 :Text;
+  }
+
+  struct JoinResponse {
+    playerId @0: UInt16;
+  }
+
+  listPlayers @1 () -> (response: List(Player));
+
+  struct Player {
+    playerId @0: UInt16;
+    did @1: Text;
+  }
+}