revert(mqtt): remove IPv6 dual-stack support — measured heap exhaustion

Reverts merge ae04539 (feat/flex-ipv6, PR #25). Enabling IPv6 joins the
device into IPv6 multicast/ND processing; on multicast-heavy LANs (e.g.
with a Thread/Matter border router advertising a ULA prefix) inbound
bursts land in dynamic WiFi RX buffers in internal heap. Measured on a
Station G2 with 4 WSS brokers: min-free floor dropped 36 KB -> 18 KB,
largest free block pinned below the 16 KB publish threshold for minutes
at a time, 523 dropped publishes in 15 minutes. With IPv6 disabled the
floor and max-alloc recovered and publish skips stopped.

The feature only fed the wifi.status display line — no transport uses
IPv6 (all brokers connect over IPv4), so the fleet risk (network-
dependent degradation on unknown home LANs) buys nothing. Revisit after
the Arduino core 3.x / IDF 5.x move if IPv6 transport is ever needed;
the branch remains at feat/flex-ipv6.

Kept from the merge: the wifi.status uptime append now computes the
actual remaining space in the 160-byte reply buffer instead of assuming
a hardcoded 128, fixing a latent overflow of the snprintf bound.

Author: agessaman

MQTT_IMPLEMENTATION.md

@@ -249,26 +249,6 @@ The MQTT bridge comes with the following defaults for fresh installs (unless ove
 - **Timezone Offset**: 0 (fallback, no offset, unless `MQTT_DEFAULT_TIMEZONE_OFFSET` is set)
 - **Repeat (forwarding)**: On (set `repeat off` for receive-only observers)
 
-## IPv6 Support
-
-Observer builds run **dual-stack**: IPv4 continues to work exactly as before, and the node
-*additionally* acquires an IPv6 address when the network supports it. This is fully automatic
-and requires no configuration.
-
-- **How it works**: once WiFi has an IPv4 address, the node enables IPv6 and obtains a global
-  address via Router Advertisement / SLAAC. A dual-stack router with RA/SLAAC is required;
-  on IPv4-only networks the node simply stays IPv4-only (graceful degradation).
-- **Visibility**: the global IPv6 address appears in `get wifi.status` once assigned, e.g.
-  `> connected, IP: 192.168.1.42, IPv6: 2001:db8::abcd, RSSI: -62 dBm, uptime: ...`.
-  The field is omitted when no global address is present (link-local is not reported). IPv6 is
-  CLI/serial-visible only — it is not shown on the OLED.
-- **Custom broker over IPv6**: use a bracketed literal in a full URI
-  (`set mqttN.server mqtts://[2001:db8::1]:8883`), or a bare literal
-  (`set mqttN.server 2001:db8::1`) which is bracketed automatically. Hostname presets need no
-  changes — DNS resolves AAAA records automatically once the stack is dual-stack.
-- **Cost**: none to budget. IPv6 is already compiled into the ESP32 Arduino lwIP that every
-  build links; enabling it at runtime only adds a couple of address slots.
-
 ## CLI Commands
 
 ### MQTT Slot Commands

src/helpers/CommonCLI_Observer.cpp

@@ -780,26 +780,7 @@ bool CommonCLI::handleObserverGetCmd(uint32_t sender_timestamp, const char* conf
       default: status_str = "unknown"; break;
     }
     if (status == WL_CONNECTED) {
-      // reply points at the caller's char[160] command buffer (see main.cpp).
-      const size_t kReplyBufSize = 160;
-      sprintf(reply, "> %s, IP: %s", status_str, WiFi.localIP().toString().c_str());
-#ifdef WITH_MQTT_BRIDGE
-      // Group IPv6 directly after IPv4 when a global/ULA address is assigned.
-      char v6[46];
-      if (MQTTBridge::getGlobalIPv6(v6, sizeof(v6))) {
-        size_t v6_len = strlen(reply);
-        if (v6_len < kReplyBufSize) {
-          snprintf(reply + v6_len, kReplyBufSize - v6_len, ", IPv6: %s", v6);
-        }
-      }
-#endif
-      // RSSI right after the IP addresses.
-      {
-        size_t rssi_len = strlen(reply);
-        if (rssi_len < kReplyBufSize) {
-          snprintf(reply + rssi_len, kReplyBufSize - rssi_len, ", RSSI: %d dBm", WiFi.RSSI());
-        }
-      }
+      sprintf(reply, "> %s, IP: %s, RSSI: %d dBm", status_str, WiFi.localIP().toString().c_str(), WiFi.RSSI());
 #ifdef WITH_MQTT_BRIDGE
       unsigned long connect_at = MQTTBridge::getWifiConnectedAtMillis();
       if (connect_at != 0) {
@@ -809,6 +790,9 @@ bool CommonCLI::handleObserverGetCmd(uint32_t sender_timestamp, const char* conf
         unsigned long h = (uptime_sec % 86400) / 3600;
         unsigned long m = (uptime_sec % 3600) / 60;
         unsigned long s = uptime_sec % 60;
+        // reply points at the caller's char[160] command buffer (see main.cpp);
+        // compute the actual remaining space instead of assuming 128.
+        const size_t kReplyBufSize = 160;
         size_t len = strlen(reply);
         const size_t reply_remaining = (len < kReplyBufSize) ? (kReplyBufSize - len) : 0;
         if (d > 0) {

src/helpers/bridges/MQTTBridge.cpp

@@ -15,7 +15,6 @@
 
 #ifdef ESP_PLATFORM
 #include <esp_wifi.h>
-#include <esp_netif.h>
 #include <esp_heap_caps.h>
 #include <freertos/FreeRTOS.h>
 #include <freertos/task.h>
@@ -180,10 +179,6 @@ static unsigned long s_wifi_connected_at = 0;
 static uint8_t s_wifi_disconnect_reason = 0;
 static unsigned long s_wifi_disconnect_time = 0;
 
-// Most recent global/unique-local IPv6 address (SLAAC), as a string; empty when none.
-// Populated from the ARDUINO_EVENT_WIFI_STA_GOT_IP6 handler; cleared on WiFi disconnect.
-static char s_global_ipv6[46] = "";
-
 #ifdef MQTT_MEMORY_DEBUG
 // #region agent log
 static void agentLogHeap(const char* location, const char* message, const char* hypothesisId,
@@ -206,17 +201,6 @@ unsigned long MQTTBridge::getWifiConnectedAtMillis() {
   return s_wifi_connected_at;
 }
 
-bool MQTTBridge::getGlobalIPv6(char* buf, size_t len) {
-  if (buf == nullptr || len == 0) return false;
-  if (s_global_ipv6[0] == '\0') {
-    buf[0] = '\0';
-    return false;
-  }
-  strncpy(buf, s_global_ipv6, len - 1);
-  buf[len - 1] = '\0';
-  return true;
-}
-
 void MQTTBridge::formatMqttStatusReply(char* buf, size_t bufsize, const MQTTPrefs* obs) {
   if (buf == nullptr || bufsize == 0) return;
   const char* msgs = (obs && obs->mqtt_status_enabled) ? "on" : "off";
@@ -845,23 +829,11 @@ void MQTTBridge::initializeWiFiInTask() {
       switch(event) {
         case ARDUINO_EVENT_WIFI_STA_GOT_IP:
           MQTT_DEBUG_PRINTLN("WiFi connected: %s", IPAddress(info.got_ip.ip_info.ip.addr).toString().c_str());
-          // Kick off IPv6 link-local + RA/SLAAC (additive; IPv4 path unchanged). Idempotent.
-          WiFi.enableIpV6();
           // Set flag to trigger NTP sync from loop() instead of doing it here
           if (!_ntp_synced && !_ntp_sync_pending) {
             _ntp_sync_pending = true;
           }
           break;
-        case ARDUINO_EVENT_WIFI_STA_GOT_IP6: {
-          // Store only global/unique-local addresses; link-local isn't useful for diagnostics.
-          esp_ip6_addr_t ip6 = info.got_ip6.ip6_info.ip;
-          esp_ip6_addr_type_t type = esp_netif_ip6_get_addr_type(&ip6);
-          if (type == ESP_IP6_ADDR_IS_GLOBAL || type == ESP_IP6_ADDR_IS_UNIQUE_LOCAL) {
-            snprintf(s_global_ipv6, sizeof(s_global_ipv6), IPV6STR, IPV62STR(ip6));
-            MQTT_DEBUG_PRINTLN("WiFi IPv6: %s", s_global_ipv6);
-          }
-          break;
-        }
         case ARDUINO_EVENT_WIFI_STA_DISCONNECTED:
           s_wifi_disconnect_reason = info.wifi_sta_disconnected.reason;
           s_wifi_disconnect_time = millis();
@@ -1353,16 +1325,7 @@ void MQTTBridge::setupSlot(int index) {
       } else if (slot.port == 443) {
         proto = "wss";
       }
-      // Bare IPv6 literals (contain ':' but no '.', not already bracketed) must be wrapped
-      // in [..] so the ":port" suffix isn't mistaken for part of the address.
-      bool bare_ipv6 = (slot.host[0] != '[') &&
-                       (strchr(slot.host, ':') != nullptr) &&
-                       (strchr(slot.host, '.') == nullptr);
-      if (bare_ipv6) {
-        snprintf(slot.broker_uri, sizeof(slot.broker_uri), "%s://[%s]:%d", proto, slot.host, slot.port);
-      } else {
-        snprintf(slot.broker_uri, sizeof(slot.broker_uri), "%s://%s:%d", proto, slot.host, slot.port);
-      }
+      snprintf(slot.broker_uri, sizeof(slot.broker_uri), "%s://%s:%d", proto, slot.host, slot.port);
     }
     slot.client->setServer(slot.broker_uri);
     MQTT_DEBUG_PRINTLN("MQTT%d custom broker URI: %s (host='%s', port=%u)",
@@ -2117,9 +2080,6 @@ bool MQTTBridge::handleWiFiConnection(unsigned long now) {
       _wifi_disconnected_time = 0;
       s_wifi_connected_at = now;
       _wifi_reconnect_backoff_attempt = 0;
-      // Re-arm IPv6 link-local after a reconnect (covers paths that re-run WiFi.begin
-      // and skip the GOT_IP event ordering). Idempotent.
-      WiFi.enableIpV6();
       #ifdef ESP_PLATFORM
       wifi_ps_type_t ps_mode;
       uint8_t ps_pref = _obs->wifi_power_save;
@@ -2146,7 +2106,6 @@ bool MQTTBridge::handleWiFiConnection(unsigned long now) {
     if (_last_wifi_status == WL_CONNECTED) {
       _wifi_disconnected_time = now;
       s_wifi_connected_at = 0;
-      s_global_ipv6[0] = '\0';  // drop stale IPv6 so get wifi.status doesn't report it
       // Disconnect all slot clients when WiFi drops
       for (int i = 0; i < RUNTIME_MQTT_SLOTS; i++) {
         if (_slots[i].client && _slots[i].connected) {

src/helpers/bridges/MQTTBridge.h

@@ -430,10 +430,6 @@ class MQTTBridge : public BridgeBase {
 
   static unsigned long getWifiConnectedAtMillis();
 
-  // Copies the current global/unique-local IPv6 address (string) into buf.
-  // Returns false (and writes an empty string) when no global IPv6 is assigned.
-  static bool getGlobalIPv6(char* buf, size_t len);
-
   /**
    * Per-slot outage accessors used by AlertReporter to detect prolonged
    * MQTT broker outages. Indices are 0..RUNTIME_MQTT_SLOTS-1.