Add real Application::loop() benchmark, call original_setup() in main

- Call original_setup() in benchmark main.cpp so code-generated App
  initialization (pre_setup, area/device registration, looping_components_
  init) runs before benchmarks
- Restore ApplicationLoop_Empty benchmark that calls the actual App.loop()

tests/benchmarks/core/bench_application_loop.cpp

@@ -0,0 +1,19 @@
+#include <benchmark/benchmark.h>
+
+#include "esphome/core/application.h"
+
+namespace esphome::benchmarks {
+
+// Benchmark Application::loop() with no registered components.
+// App is initialized by original_setup() in main.cpp (code-generated
+// pre_setup, area/device registration, looping_components_.init).
+// This measures the baseline overhead of the main loop: scheduler,
+// timing, before/after loop tasks, and yield_with_select_.
+static void ApplicationLoop_Empty(benchmark::State &state) {
+  for (auto _ : state) {
+    App.loop();
+  }
+}
+BENCHMARK(ApplicationLoop_Empty);
+
+}  // namespace esphome::benchmarks