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

task.md

@@ -166,6 +166,26 @@ Shipped. What landed vs the plan:
   `HashMap<String,Pid>` in `SharedState`.
 - gen_server-style call/cast: request-reply correlation as a thin layer over
   channels (`call` sends `{req, reply_tx}`, awaits `reply_rx`); no runtime change.
+  ✅ DONE (`a4fcf6c`). What landed vs the plan:
+  - `GenServer` trait on the state value: assoc `Call`/`Reply`/`Cast` types,
+    required `handle_call`/`handle_cast`, optional `init`/`terminate` hooks.
+    `ServerRef<G>` is a clonable inbox sender + `pid()`; `start` / `start_under`.
+  - One inbox, not two: a single `Envelope { Call(req, reply_tx) | Cast }`
+    channel, dispatched by variant. Forced by no-`select`/no-unified-mailbox —
+    a server can't wait on a call channel and a cast channel at once.
+  - Server-down falls out of channel closure (no monitor needed): `send` fails
+    if the inbox is gone; the reply sender drops on the server's unwind so a
+    parked caller wakes to `Err`. Both → `Call/CastError::ServerDown`.
+  - `terminate` runs via a drop guard → fires on *every* exit path (clean inbox
+    close, handler panic, `request_stop`), not just the clean one. Caveat: it
+    may run mid-unwind, so keep it non-blocking (a panic inside it during an
+    unwind double-panics → abort).
+  - No `handle_info`, no call timeout — both deferred to land with timeouts
+    (`handle_info` needs the still-unmade cross-channel mailbox merge; a call
+    timeout needs a per-`recv` deadline / `Signal::Timeout`).
+  - Pure additive layer (no Slot/scheduler/spawn/finalize change) → no perf
+    check run. Tests (`tests/gen_server.rs`): cast→call roundtrip, init/terminate
+    ordering, both server-down paths.
 - Docs lag: README still says "x86-64 Linux only — ARM64 … deferred" and the
   module table calls `context` x86-64-only; update for the aarch64 backend.