docs,examples: graceful shutdown — new examples/graceful_shutdown.rs, README 'Stopping actors', named_genserver uses shutdown(), Deep Dive terminate note, ROADMAP open items

ROADMAP.md

@@ -272,6 +272,21 @@ outright — `join` what you need finished. No forcing sweep follows.
 
 ---
 
+### Open items from the graceful-shutdown work (not scheduled)
+- **gen_server / gen_statem as a direct supervised child.** A `ChildSpec` start
+  fn is `Fn()`, and `GenServerBuilder::start` spawns a *new* actor and hands
+  back a ref, so a supervised server today needs a trapping wrapper actor that
+  starts it `under(self_pid())`, forwards the shutdown and waits
+  (`examples/graceful_shutdown.rs::drainer_child`). An inline
+  `GenServerBuilder::run()` (loop as the current actor; ref handed out via a
+  name or a start callback) would make the primary OTP use case direct. Needed
+  by urus's endpoint child.
+- Supervisor `Live` drop-guard sweep is `request_stop` (kill propagates as
+  kill); OTP would deliver a trappable `killed`. Chosen for boundedness.
+- A root-exit shutdown reaches only actors live *at that instant*; a
+  non-trapping forest root that spawns before it unwinds leaves that spawn
+  to itself (Erlang: an unlinked spawn is nobody's child).
+
 ## Invariants & gotchas (respect these across all cycles)
 
 - **Shared mutex is non-reentrant.** `Sender::send` can call `unpark` →