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.

README.md

@@ -69,6 +69,14 @@ root actor returning means "the program is done": every top-level actor gets a
 runtime (a signal thread), `Runtime::handle().request_shutdown(pid)` does the
 same. `examples/graceful_shutdown.rs` shows all of it.
 
+A gen_server or gen_statem lives until it stops, is shut down, or is killed; its
+refs are addresses — dropping them never ends it (a forgotten one is swept at
+root exit; with `--features smarm-trace` each such sweep is a `root_sweep`
+trace line). The supervised shape is `GenServerBuilder::named(N).run()` /
+`gen_statem::run_named(N, m)`: the server runs inline as the `ChildSpec` child
+itself, so the supervisor's shutdown reaches it directly, a restart re-binds the
+name, and the program addresses it by name.
+
 ## Layout
 
 ```