feat(firmware): event-driven WiFi reconnect with exponential backoff

net_guard registers WiFi.onEvent() so disconnects are handled
immediately instead of polled every 1s. Backoff 1s->2s->4s->...->60s cap.
Every up/down transition is logged to the event log with the disconnect
reason code, so field failures are diagnosable.

firmware/lib/net_guard/net_guard.cpp

@@ -0,0 +1,55 @@
+// firmware/lib/net_guard/net_guard.cpp
+#include "net_guard.h"
+
+uint32_t net_guard_next_backoff_ms(uint32_t attempt) {
+    if (attempt >= 6) return 60000;
+    return 1000u * (1u << attempt);
+}
+
+#ifdef ARDUINO
+#include <WiFi.h>
+#include "event_log.h"
+
+static const DeviceConfig* s_cfg = nullptr;
+static volatile uint8_t    s_last_disconnect = 0;
+static volatile bool       s_up = false;
+static volatile uint32_t   s_attempts = 0;
+static volatile uint32_t   s_next_retry_ms = 0;
+
+static void on_wifi_event(WiFiEvent_t event, WiFiEventInfo_t info) {
+    switch (event) {
+        case ARDUINO_EVENT_WIFI_STA_GOT_IP:
+            s_up = true;
+            s_attempts = 0;
+            s_next_retry_ms = 0;
+            event_log_write(EVT_WIFI_UP, (uint16_t)(int16_t)WiFi.RSSI(), 0);
+            break;
+        case ARDUINO_EVENT_WIFI_STA_DISCONNECTED:
+            s_up = false;
+            s_last_disconnect = (uint8_t)info.wifi_sta_disconnected.reason;
+            event_log_write(EVT_WIFI_DOWN, s_last_disconnect, 0);
+            s_next_retry_ms = millis() + net_guard_next_backoff_ms(s_attempts);
+            break;
+        default: break;
+    }
+}
+
+void net_guard_start(const DeviceConfig& cfg) {
+    s_cfg = &cfg;
+    WiFi.onEvent(on_wifi_event);
+    WiFi.setAutoReconnect(false); // we drive reconnect ourselves
+}
+
+bool net_guard_is_up() { return s_up; }
+
+uint8_t net_guard_last_disconnect_reason() { return s_last_disconnect; }
+
+extern "C" void net_guard_tick() {
+    if (s_up || s_cfg == nullptr) return;
+    if (millis() < s_next_retry_ms) return;
+    s_attempts++;
+    WiFi.disconnect(false, false);
+    WiFi.begin(s_cfg->wifi_ssid.c_str(), s_cfg->wifi_pass.c_str());
+    s_next_retry_ms = millis() + net_guard_next_backoff_ms(s_attempts);
+}
+#endif

firmware/lib/net_guard/net_guard.h

@@ -0,0 +1,24 @@
+// firmware/lib/net_guard/net_guard.h
+#pragma once
+#include <stdint.h>
+
+// Exponential backoff: 1s, 2s, 4s, 8s, 16s, 32s, 60s, 60s, ...
+// attempt 0 -> 1000ms, clamped at 60000ms.
+uint32_t net_guard_next_backoff_ms(uint32_t attempt);
+
+#ifdef ARDUINO
+#include "config.h"
+
+// Registers WiFi.onEvent() handler and starts auto-reconnect loop.
+// Must be called once after WiFi.begin() succeeds.
+void net_guard_start(const DeviceConfig& cfg);
+
+// True iff WiFi is currently associated with IP.
+bool net_guard_is_up();
+
+// Last disconnect reason code from WIFI_EVENT_STA_DISCONNECTED (0 = none).
+uint8_t net_guard_last_disconnect_reason();
+
+// Non-blocking tick called from loop(); kicks reconnect if due.
+extern "C" void net_guard_tick();
+#endif