docs: mark links/trap_exit (#3) done; refresh supervisor + README

- task.md: mark roadmap #3 done (record the resolved dedicated-inbox
  decision + test list), add the feature commit to the resume block, note
  the now-resolved trap_exit channel question, and list spawn_link under #5.
- supervisor.rs: the module comment predated cancellation; rewrite it to say
  sibling/link/shutdown stops are cooperative (request_stop), not that
  one_for_all / rest_for_one / links are impossible.
- README: add monitor + link rows, refresh the supervisor row and src layout,
  and drop restart-intensity caps from "what's not here" (shipped in #2).

README.md

@@ -25,7 +25,9 @@ convenience wrapper around `runtime::init(Config::exact(1)).run(f)`.
 | `mutex`      | `Mutex<T>` with mandatory timeout; FIFO waiters; parks the green thread |
 | `timer`      | Min-heap of `(deadline, reason)`; `Sleep` and `WaitTimeout` reasons    |
 | `io`         | `block_on_io` for blocking work; `wait_readable`/`wait_writable` + `read`/`write` via epoll |
-| `supervisor` | `Signal::Exit` / `Signal::Panic` delivered to a parent actor's mailbox |
+| `supervisor` | `Signal::Exit`/`Panic`/`Stopped` funnelled to a parent; `OneForOne`/`OneForAll`/`RestForOne` strategies + restart-intensity cap |
+| `monitor`    | `monitor(pid)` → one-shot `Receiver<Down>`; unidirectional death notice |
+| `link`       | bidirectional `link`/`unlink`; abnormal death propagates (cooperative stop, or an `ExitSignal` message under `trap_exit`) |
 
 ## Quick taste
 
@@ -52,7 +54,8 @@ run(|| {
 ```
 src/
   stack.rs context.rs preempt.rs pid.rs actor.rs
-  scheduler.rs channel.rs mutex.rs timer.rs io.rs supervisor.rs
+  scheduler.rs channel.rs mutex.rs timer.rs io.rs
+  supervisor.rs monitor.rs link.rs runtime.rs
   lib.rs
 tests/
   per-module integration tests
@@ -76,7 +79,7 @@ cargo bench               # primes benchmark vs tokio
 ## What's not here
 
 See the **Defer** section of `Architecture.md`. 
-restart-intensity caps, `join!` for handle groups, stack growth via remap,
+`join!` for handle groups, stack growth via remap,
 hierarchical timer wheel, fd-wait timeouts, `Signal::Timeout`. Each is
 mechanism we know how to add; none belongs in this iteration.