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).

src/supervisor.rs

@@ -51,13 +51,16 @@ impl Signal {
 // whether to restart, and enforce a restart-intensity cap so a child that
 // crashes in a tight loop eventually gives up instead of spinning forever.
 //
-// What this deliberately does NOT do: forcibly terminate a running child.
-// smarm actors share a heap and rely on Drop/RAII, so async teardown of a
-// peer's stack from outside is unsound. That rules out `one_for_all` /
-// `rest_for_one` (which must stop siblings) and true bidirectional links
-// until there is a cooperative-cancellation primitive. When the intensity cap
-// trips, this supervisor simply stops restarting and returns; it does not kill
-// whatever children are still alive.
+// What this does NOT do: *forcibly* terminate a running child. smarm actors
+// share a heap and rely on Drop/RAII, so tearing down a peer's stack from
+// outside is unsound. Stopping a sibling — whether for `one_for_all` /
+// `rest_for_one`, for a propagated link death, or for the ordered shutdown
+// below — is therefore *cooperative*: `request_stop` flags the child and it
+// unwinds at its next observation point. A child with no observation points
+// (a tight loop with no `check!()`, allocation, or blocking op) cannot be
+// stopped, exactly as it cannot be preempted. When the intensity cap trips,
+// this supervisor stops restarting and tears the remaining children down in
+// reverse start order before returning.
 // ---------------------------------------------------------------------------
 
 use crate::channel::channel;