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>

test/mocks/config.h

@@ -13,6 +13,7 @@
 #define SLEEP_CLAMP_MIN_MS         30000ULL
 #define SLEEP_CLAMP_MAX_MS         (25ULL * 60ULL * 60ULL * 1000ULL)
 #define FIRST_IMAGE_POLL_INTERVAL_MS 15000ULL
+#define BOOTSTRAP_RETRY_INTERVAL_MS  2000ULL
 #define WIFI_TIMEOUT_MS   30000
 #define RESET_HOLD_MS     5000
 #define AP_IP             "192.168.4.1"