docs(roadmap): mark gen_server done; add module to README

README.md

@@ -28,6 +28,7 @@ convenience wrapper around `runtime::init(Config::exact(1)).run(f)`.
 | `supervisor` | `Signal::Exit`/`Panic`/`Stopped` funnelled to a parent; `OneForOne`/`OneForAll`/`RestForOne` strategies + restart-intensity cap |
 | `monitor`    | `monitor(pid)` → one-shot `Receiver<Down>`; unidirectional death notice |
 | `link`       | bidirectional `link`/`unlink`; abnormal death propagates (cooperative stop, or an `ExitSignal` message under `trap_exit`) |
+| `gen_server` | `call` (sync request-reply) / `cast` (async) over one inbox; `ServerRef` + `init`/`terminate` hooks; server-down via channel closure |
 
 ## Quick taste
 
@@ -56,7 +57,7 @@ src/
   stack.rs context.rs preempt.rs pid.rs actor.rs
   scheduler.rs channel.rs mutex.rs timer.rs io.rs
   supervisor.rs monitor.rs link.rs runtime.rs
-  lib.rs
+  gen_server.rs lib.rs
 tests/
   per-module integration tests
 benches/