[core] Add CodSpeed C++ benchmarks for protobuf and main loop

Add automated benchmarks using Google Benchmark to prevent performance
regressions in the API protobuf encoding/decoding and core loop paths.

Benchmarks cover:
- Protobuf encode: SensorState, BinarySensorState, HelloResponse,
  LightState, DeviceInfoResponse (20 nested devices + 20 areas)
- Protobuf decode: HelloRequest, SwitchCommand, LightCommand
- Protobuf calculate_size and full calc+encode send path
- Varint parse/encode/size for various value ranges
- Scheduler call/next_schedule_in with idle and active timers
- Application loop component dispatch and blocking guard overhead

Infrastructure:
- Extract shared build logic from cpp_unit_test.py into test_helpers.py
- Add cpp_benchmark.py mirroring the unit test build pattern
- Support benchmark.yaml per component dir for declaring dependencies
- Add CodSpeed CI workflow triggered on api/core changes
- Fix ProtoMessage protected destructor on host platform

tests/benchmarks/components/main.cpp

@@ -0,0 +1,38 @@
+#include <benchmark/benchmark.h>
+
+#include "esphome/components/logger/logger.h"
+
+/*
+This special main.cpp provides the entry point for Google Benchmark.
+It replaces the default ESPHome main with a benchmark runner.
+See the codspeed_plan.md for more information.
+*/
+
+// Auto generated code by esphome
+// ========== AUTO GENERATED INCLUDE BLOCK BEGIN ===========
+// ========== AUTO GENERATED INCLUDE BLOCK END ==========="
+
+void original_setup() {
+  // This function won't be run.
+
+  // ========== AUTO GENERATED CODE BEGIN ===========
+  // =========== AUTO GENERATED CODE END ============
+}
+
+void setup() {
+  // Log functions call global_logger->log_vprintf_() without a null check,
+  // so we must set up a Logger before any test that triggers logging.
+  static esphome::logger::Logger test_logger(0);
+  test_logger.set_log_level(ESPHOME_LOG_LEVEL);
+  test_logger.pre_setup();
+
+  int argc = 1;
+  char arg0[] = "benchmark";
+  char *argv[] = {arg0, nullptr};
+  ::benchmark::Initialize(&argc, argv);
+  ::benchmark::RunSpecifiedBenchmarks();
+  ::benchmark::Shutdown();
+  exit(0);
+}
+
+void loop() {}