[mdns] Tighten ESP8266 guards, expand window to 15s, document derivation

- mdns_esp8266.cpp: narrow setup()'s listener block and on_ip_state from
  USE_MDNS_EVENT_DRIVEN_POLLING to USE_MDNS_WIFI_LISTENER. ESP8266 has no
  ethernet so the practical effect is identical, but the tighter guard is
  symmetric with mdns_rp2040.cpp and more defensible if the listener macros
  ever drift.
- MDNS_POLL_WINDOW_MS 12000 → 15000. The RFC-compliant LEAmDNS phase is ~9s
  worst case, but a blocked main loop (long component setup, WiFi scan, flash
  writes) can stretch the deadlines between our polls. 6s of margin absorbs
  that without materially extending the steady-state quiet period.
- Header comment for MDNS_POLL_WINDOW_MS now breaks down the 9s derivation
  with the exact LEAmDNS constants (MDNS_PROBE_DELAY × MDNS_PROBE_COUNT +
  MDNS_ANNOUNCE_DELAY × MDNS_ANNOUNCE_COUNT + jitter + debounce hop) so
  future library bumps can be justified against the same math.

esphome/components/mdns/mdns_component.h

@@ -76,7 +76,17 @@ class MDNSComponent final : public Component
   // and update() becomes pure overhead. We arm a bounded polling window from IP state
   // listener events so update() runs only during that phase.
   static constexpr uint32_t MDNS_UPDATE_INTERVAL_MS = 50;
-  static constexpr uint32_t MDNS_POLL_WINDOW_MS = 12000;  // ~9s phase + jitter/restart margin
+  // Must exceed LEAmDNS's longest restart-to-announce-complete path:
+  //   MDNS_PROBE_DELAY (250ms) × MDNS_PROBE_COUNT (3) = 750ms probing
+  // + MDNS_ANNOUNCE_DELAY (1000ms) × MDNS_ANNOUNCE_COUNT (8) = 8000ms announcing
+  // + rand() % MDNS_PROBE_DELAY jitter on first probe (0–250ms)
+  // + debounced schedule_function() hop when statusChangeCB fires on ESP8266
+  // ≈ 9s nominal. 15s gives ~6s margin to absorb main-loop blocking (long
+  // component setup, WiFi scan, flash writes) that could stretch the deadlines
+  // between our polls. If LEAmDNS ever extends its phase (upstream library
+  // update) this constant needs to grow. Constants defined in LEAmDNS_Priv.h
+  // (ESP8266 core 3.1.2 / arduino-pico 5.5.1).
+  static constexpr uint32_t MDNS_POLL_WINDOW_MS = 15000;
   static constexpr uint32_t MDNS_POLL_ID = 0;
   static constexpr uint32_t MDNS_POLL_STOP_ID = 1;
 #endif

esphome/components/mdns/mdns_esp8266.cpp

@@ -48,7 +48,7 @@ void MDNSComponent::start_polling_window_() {
 
 void MDNSComponent::setup() {
   this->setup_buffers_and_register_(register_esp8266);
-#ifdef USE_MDNS_EVENT_DRIVEN_POLLING
+#ifdef USE_MDNS_WIFI_LISTENER
   // LEAmDNS's own LwipIntf::statusChangeCB drives _restart() on netif changes; we just
   // arm the window around the initial probe/announce and each reconnect. Unconditional
   // here is safe: setup_priority::AFTER_CONNECTION guarantees the network is up.
@@ -57,7 +57,7 @@ void MDNSComponent::setup() {
 #endif
 }
 
-#ifdef USE_MDNS_EVENT_DRIVEN_POLLING
+#ifdef USE_MDNS_WIFI_LISTENER
 void MDNSComponent::on_ip_state(const network::IPAddresses &ips, const network::IPAddress &,
                                 const network::IPAddress &) {
   // IP listener only fires on acquisition (not loss), so any notification is a fresh