Merge fix/mqtt-review-2026-09-09 into feat/abstract-networking

#54 lands first, so the network abstraction is reworked on top of the
per-slot client ownership model instead of alongside it.

Two resolutions are semantic, not textual:

- Link-transition teardown goes through the ownership API. It called
  softDisconnect() on every started client, which bypassed client_state and
  ignored the typed result. A slot that is still Starting now gets a real
  stop: softDisconnect() returns immediately on a client that is not yet
  connected, so its attempt would otherwise complete against the old route and
  deliver a CONNECTED event indistinguishable from the new one's (F04). On a
  medium switch the old route can still be briefly usable, so that is not
  hypothetical. A connected client keeps the cheap bounded path, and a
  quarantined one is left alone.

- NetworkLink::applyPowerPrefs() adopts WifiPowerSavePolicy. Its local
  `2 ? MAX : NONE` mapping would have reintroduced F11 and read the new stored
  value 3 (explicit `min`) as none. It also applies the setting when the link
  starts already associated, which is the case the bridge used to cover.

The NTP probe from #54 keeps its validation and its one-attempt-per-server
bound, but resolves and gates on the selected link rather than on WiFi, so it
works on an Ethernet-preferred node.

src/helpers/esp32/WebConfigServer.cpp

@@ -17,6 +17,7 @@
 #include <helpers/MQTTPacketFilter.h>
 #include <helpers/MQTTPresets.h>
 #include <helpers/WebConfigKeys.h>
+#include <helpers/WifiPowerSavePolicy.h>
 #include <helpers/bridges/MQTTBridge.h>
 
 #include "HttpPort80Lease.h"
@@ -881,8 +882,7 @@ void WebConfigServer::handleConfigGet(AsyncWebServerRequest* req) {
     JsonObject wifi = doc.createNestedObject("wifi");
     wifi["ssid"] = (const char*)_obs->wifi_ssid;
     wifi["pwd"] = _obs->wifi_password[0] ? SECRET_SENTINEL : "";
-    wifi["powersave"] = _obs->wifi_power_save == 0 ? "min"
-                        : _obs->wifi_power_save == 2 ? "max" : "none";
+    wifi["powersave"] = WifiPowerSavePolicy::nameFor(_obs->wifi_power_save);
 
     JsonObject mqtt = doc.createNestedObject("mqtt");
     mqtt["origin"] = (const char*)_obs->mqtt_origin;