test(firmware): broaden native-test coverage + gcov instrumentation

Extract the pre-first-image retry from normal_operation() into a
templated bootstrap_loop() helper in operation.h so the loop body
becomes unit-testable. Add four tests against it: two that verify the
X-Panel-Id header is sent on every poll and matches the compile-time
PANEL_ID (a silent server-side mis-routing risk if dropped), and two
that exercise the loop's exit-on-deep-sleep vs. iterate-while-204
behaviour.

Wire --coverage into env:native-test (compile + link via a post-script)
so `gcovr -r . --filter src/` produces a real number, and ignore the
stray *.gcov files gcovr drops at the repo root.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main.cpp

@@ -292,13 +292,13 @@ static void normal_operation(const String& mac) {
     // we've received our first image. While in the pre-image window it
     // returns instead, so we keep WiFi up and retry on a short interval —
     // way faster end-to-end than waiting through a deep-sleep + reconnect
-    // for every "no image yet" poll.
-    while (true) {
+    // for every "no image yet" poll. The loop body is extracted into
+    // bootstrap_loop() (operation.h) so it's unit-testable.
+    bootstrap_loop([&]() {
         HTTPClient http;
         http.begin(client, url);
         normal_operation_impl(mac, http, url, prefs);
-        delay(BOOTSTRAP_RETRY_INTERVAL_MS);
-    }
+    });
 }
 
 // ── Setup ─────────────────────────────────────────────────────────────────────

src/operation.h

@@ -356,3 +356,23 @@ void normal_operation_impl(const String& mac, HTTP& http, const String& url, Pre
     esp_sleep_enable_ext0_wakeup((gpio_num_t)PIN_BTN_RESET, 0);
     esp_deep_sleep_start();
 }
+
+// ── Bootstrap-stay-awake loop ────────────────────────────────────────────────
+// Wraps normal_operation_impl in a retry loop for the pre-first-image window.
+// Once an image arrives, impl calls esp_deep_sleep_start() and never returns —
+// in production that's literal silicon-level halt; in unit tests the mock just
+// sets g_deep_sleep_started and returns, so we check that flag to break out.
+// Caller passes a callable that runs one poll iteration (instantiates a fresh
+// HTTPClient bound to the WiFiClient, calls normal_operation_impl, etc.).
+template<typename PollOnce>
+inline void bootstrap_loop(PollOnce poll_once) {
+    while (true) {
+        poll_once();
+#ifdef UNIT_TEST
+        // Production: esp_deep_sleep_start never returns. Tests: it sets the
+        // flag and returns, so without this guard the loop spins forever.
+        if (g_deep_sleep_started) return;
+#endif
+        delay(BOOTSTRAP_RETRY_INTERVAL_MS);
+    }
+}