feat(gen_server,gen_statem): lifetime is the actor's — refs are addresses; inline named run

Root cause behind the "pin the endpoint" gotcha and the trapping-wrapper
pattern: a gen_server had two lifetime authorities — its refs (last one
dropped → inbox closes → exit) and, when supervised, its supervisor. OTP has
one: a process lives until it stops, is shut down, or is killed; a pid is an
address. Root exit now shutting down every forest root removes the reason the
ref-governed idiom existed (a forgotten server no longer hangs the run), so
adopt the one rule:

- The server/machine loop holds one inbox sender for its life; the inbox
  never closes. GenServerRef / GenStatemRef are addresses. Explicit close is
  `shutdown()`; a forgotten one is swept at root exit.
- `NamedGenServerBuilder::run()` / `gen_statem::run_named(name, m)` run the
  loop inline as the current actor: a server is a direct ChildSpec child,
  gets the supervisor's shutdown as handle_shutdown / a shutdown row, re-binds
  its name on restart, and is addressed by name. The wrapper in
  examples/graceful_shutdown.rs is gone.
- gen_statem gains GenStatemName + whereis_machine/send/call/shutdown by name
  (parity with gen_server); the macro gets `Sm::new`.
- Root-exit sweep records `Event::RootSweep { target, trapping }` under
  smarm-trace ("root_sweep shutdown|stopped"): unsupervised leftovers are
  visible rather than silently owned-by-refs.
- Named start() name-clash path stops the spawned actor instead of relying
  on ref drop.

Tests: tests/gen_server_lifetime.rs, tests/gen_statem_lifetime.rs,
tests/root_sweep_trace.rs (feature-gated); three existing tests that used
drop-closes-inbox now use shutdown(). Docs/README/ROADMAP/Deep Dive updated.

ROADMAP.md

@@ -273,14 +273,11 @@ 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.
+- ~~gen_server / gen_statem as a direct supervised child.~~ Done: lifetime is
+  the actor's (refs are addresses, the loop holds an inbox sender);
+  `NamedGenServerBuilder::run` / `gen_statem::run_named` run the loop inline as
+  the `ChildSpec` child; the root-exit sweep traces each leftover as
+  `root_sweep` under `smarm-trace`.
 - 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