refactor(firmware): drive WiFi reconnect from net_guard events

loop() no longer blocks for 5s after a disconnect; reconnect is
scheduled from the WiFi event handler with exponential backoff.
Buffered reports flush on every clean UP transition.

firmware/lib/net_guard/library.json

@@ -0,0 +1,6 @@
+{
+  "name": "net_guard",
+  "build": {
+    "flags": ["-I$PROJECT_SRC_DIR"]
+  }
+}

firmware/src/main.cpp

@@ -9,6 +9,7 @@
 #include "ble_scanner.h"
 #include "reporter.h"
 #include "event_log.h"
+#include "net_guard.h"
 #include <esp_system.h>
 
 // LED on GPIO2 (TimerCamera-F built-in LED) — verify against board schematic
@@ -172,6 +173,7 @@ void setup() {
         ESP.restart();
     }
 
+    net_guard_start(g_cfg);
     led_set(false); // off = connected
 
     // NTP sync (UTC)
@@ -208,15 +210,14 @@ void setup() {
 void loop() {
     ArduinoOTA.handle();
     check_factory_reset();
+    net_guard_tick();
 
-    if (WiFi.status() != WL_CONNECTED) {
-        led_set(true); // on = no WiFi
-        WiFi.reconnect();
-        delay(5000);
-        if (WiFi.status() == WL_CONNECTED) {
-            led_set(false);
-            reporter_flush(g_cfg);
-        }
+    static bool s_was_up = true;
+    bool up = net_guard_is_up();
+    if (up != s_was_up) {
+        led_set(!up); // LED on when NOT up
+        if (up) reporter_flush(g_cfg);
+        s_was_up = up;
     }
-    delay(1000);
+    delay(200);
 }