docs+examples: v0.3 endpoint API; crud converted to the app-owns-the-tree shape

- crud is now the demonstrator: app owns smarm runtime + root supervisor,
  store actor and urus::endpoint as ordered siblings (store first, so
  reverse-order shutdown drains HTTP before stopping the store),
  Shutdown::Infinity on the endpoint, shutdown via
  rt.handle().request_shutdown(root_sup) from the stdin thread.
  Deletes two kludges the old shape forced:
    * static OnceLock<Sender> spawn-on-first-use -> a supervised child
      that self-registers a typed Name; handlers use smarm::send per
      request and turn 'between incarnations' into a 503 instead of
      panicking on a dropped store.
    * static SHUTTING_DOWN AtomicBool + 250ms recv_timeout poll in the
      store loop and in the SSE ticker -> a plain park; the tree stops
      both. Smoke-tested live: CRUD round-trips, SSE stream, clean drain
      with the stream open, port closed after.
- Other examples stay short and on serve*, updated for the split config
  (serve_with(cfg, smarm::Config, pipe) / serve_with_shutdown(..., signal)).
  plain_serve's URUS_SCHED_THREADS now builds a smarm::Config.
  ws_chat's doc block explains the OnceLock is a serve*-only workaround
  and points at crud for the clean shape.
- README: new 'Your Own Supervision Tree' section (endpoint as the real
  API), graceful shutdown reframed as the serve*-only path, Config table
  loses scheduler_threads and gains name, PubSub rule 1 notes the
  supervised-sibling alternative.

111 lib + 50 integration + 2 doc tests green; clippy clean.

examples/channels_chat.rs

@@ -111,7 +111,12 @@ fn main() {
         handle.shutdown();
     });
 
-    serve_with_shutdown(Config::new("0.0.0.0:8080".parse().unwrap()), Pipeline::new().plug(router), signal)
+    serve_with_shutdown(
+        Config::new("0.0.0.0:8080".parse().unwrap()),
+        smarm::Config::default(),
+        Pipeline::new().plug(router),
+        signal,
+    )
         .unwrap();
     println!("channels_chat: drained, bye");
 }