fix: close Audio Router self-echo PUBACK/subscription-delivery race

publish_router_mode registered its self-echo correlation marker before
publishing but removed it in a finally block the instant the QoS 1 publish()
call observed its PUBACK. A broker may legally deliver a Controller's own
subscription copy of that same publish after the PUBACK, since MQTT does not
order the two; when that happened, the Controller mistook its own late echo
for live operator intent and relinquished automatic restoration ownership.

Keep the marker claimable for a short, bounded mqtt.router_echo_grace_seconds
window (default 2s) after the PUBACK instead of clearing it immediately.
Genuine external/operator commands are unaffected: once the grace window
elapses, a matching payload is still correctly treated as operator intent
and ownership is relinquished as before.

Verified with deterministic fake-clock unit tests plus a real Mosquitto
broker integration test (tests/test_router_echo_mosquitto_integration.py,
skipped automatically when Docker is unavailable).

README.md

@@ -345,8 +345,12 @@ when all of these remain true:
 - No live, non-self mode command and no conflicting state change indicated an
   operator override.
 
-Controller subscribes to both Router mode set and state topics. Its exact
-in-flight echo is recognized; every other live mode-set publication is treated
+Controller subscribes to both Router mode set and state topics. Its own
+mode-set publication is correlated by exact payload and stays recognized as
+a self-echo both while the publish is in flight and, because a broker may
+legally deliver a subscription echo after the PUBACK that acknowledged the
+same publish, for a short `mqtt.router_echo_grace_seconds` window afterward
+(default 2 seconds). Every other live mode-set publication is treated
 conservatively as operator intent, even if it repeats `video`. A retained set
 message is ignored as stale intent. Controller does not publish Router state;
 it accepts a received live or retained state message as the Router's current