No legacy listener can be added while the switch is off (LP-4)

While legacy mail protocols are off, x:NetworkListener/set refuses to
create a listener the switch would close, and refuses an update that would
turn an existing one into such a listener -- otherwise changing a
listener's protocol would walk straight past the check. The refusal is
invalidProperties on protocol (or on bind, for a submission listener once
SMTP is unlocked, since its port is what makes it one), and its description
names inbuxa:ProtocolPolicy and says to turn legacy protocols back on first.

The rule is the switch's own, listeners::closes, so what can't be added is
exactly what the switch would close: locked protocols (SMTP, LMTP, HTTP)
and the inbound port are never refused. Putting saved listeners back
(LP-5) writes through the registry, not /set, so it is unaffected.

The e2e changes with it. LP-6's check that an IMAP listener "created by
mistake" refuses sign-in can't be set up any more -- LP-4 is what stops
that listener existing -- so that step now proves test 4 instead: creating
an IMAP listener is refused, naming the policy; an SMTP listener can still
be created; and updating it to IMAP is refused. LP-6 stays proven live over
submission, and its IMAP wording by unit tests. All checks pass.

crates/jmap/src/registry/set.rs

@@ -502,6 +502,11 @@ impl RegistrySet for Server {
                             )
                             .await?
                         }
+                        // inbuxa: legacy-protocols LP-4
+                        ObjectInner::NetworkListener(listener) => {
+                            crate::inbuxa::protocol_policy::validate_listener(&set, listener)
+                                .await?
+                        }
                         // inbuxa: ME-12 to ME-17
                         ObjectInner::MaskedEmail(mask) => {
                             let old = match &modification {