Listeners can be stopped one at a time, which LP-2 needs

The legacy-protocols switch has to close the IMAP, POP3 and ManageSieve
ports and leave everything else accepting. The server could not do that.

Two findings from the source, both now recorded in the spec. A settings
reload never closes a port: cache/reload.rs parses the listeners only to
collect configuration errors and drops the result, and sockets are bound
once at startup through init.servers.spawn in main.rs. And there is only
one shutdown signal -- Listeners::spawn makes a single watch channel and
hands every listener a clone -- so the one thing the server could do was
stop all of them at once, port 25 included. That answers the spec's open
question 1, and the answer was neither of the two it offered.

So each listener gets its own channel. ListenerControl holds the sending
ends keyed by listener id; firing one breaks that accept loop, which drops
its TcpListener and closes the socket. The accept loop itself is unchanged
-- it already did the right thing, it just had no way to be told about one
listener. stop_matching takes a predicate and a keep list, because the
inbound listener shares its protocol with submission and telling them
apart is the caller's job (LP-3), not this registry's.

spawn_with_control is a second method rather than a change to spawn. The
registry owns the senders, so a dropped registry would stop every listener
at once; the four test callers pass no registry and keep the old shared
channel exactly as it was.

Whole-server shutdown now fires the per-listener channels too, since the
returned sender no longer reaches them.

No policy, no JMAP and no screen yet: this is only the mechanism, with
seven tests over stopping one, stopping many, sparing port 25 and sparing
submission. It closes no port on its own, and it does not touch the host's
firewall or any port-forward -- that is LP-20, and stays the operator's.

crates/common/src/config/inner.rs

@@ -67,6 +67,7 @@ impl Data {
 
         Data {
             spam_classifier: ArcSwap::from_pointee(SpamClassifier::default()),
+            listener_control: Default::default(),
             tls_certificates: ArcSwap::from_pointee(certificates),
             tls_self_signed_cert: build_self_signed_cert(
                 subject_names
@@ -222,6 +223,7 @@ impl Default for Data {
     fn default() -> Self {
         Self {
             spam_classifier: Default::default(),
+            listener_control: Default::default(),
             tls_certificates: Default::default(),
             tls_self_signed_cert: Default::default(),
             blocked_ips: Default::default(),