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/test/test_net_guard/test_net_guard.cpp

@@ -0,0 +1,32 @@
+// firmware/test/test_net_guard/test_net_guard.cpp
+#include <unity.h>
+#include "net_guard.h"
+
+void setUp()    {}
+void tearDown() {}
+
+void test_backoff_starts_at_one_second() {
+    TEST_ASSERT_EQUAL(1000, net_guard_next_backoff_ms(0));
+}
+
+void test_backoff_doubles_each_attempt() {
+    TEST_ASSERT_EQUAL(2000,  net_guard_next_backoff_ms(1));
+    TEST_ASSERT_EQUAL(4000,  net_guard_next_backoff_ms(2));
+    TEST_ASSERT_EQUAL(8000,  net_guard_next_backoff_ms(3));
+    TEST_ASSERT_EQUAL(16000, net_guard_next_backoff_ms(4));
+    TEST_ASSERT_EQUAL(32000, net_guard_next_backoff_ms(5));
+}
+
+void test_backoff_clamps_at_60s() {
+    TEST_ASSERT_EQUAL(60000, net_guard_next_backoff_ms(6));
+    TEST_ASSERT_EQUAL(60000, net_guard_next_backoff_ms(7));
+    TEST_ASSERT_EQUAL(60000, net_guard_next_backoff_ms(100));
+}
+
+int main() {
+    UNITY_BEGIN();
+    RUN_TEST(test_backoff_starts_at_one_second);
+    RUN_TEST(test_backoff_doubles_each_attempt);
+    RUN_TEST(test_backoff_clamps_at_60s);
+    return UNITY_END();
+}