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"]
+  }
+}