[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/api/bench_proto_varint.cpp

@@ -0,0 +1,99 @@
+#include <benchmark/benchmark.h>
+
+#include "esphome/components/api/proto.h"
+#include "esphome/components/api/api_buffer.h"
+
+namespace esphome::api::benchmarks {
+
+// --- ProtoVarInt::parse() benchmarks ---
+
+static void BM_ProtoVarInt_Parse_SingleByte(benchmark::State &state) {
+  // Single-byte varint (0-127) — the most common case (fast path)
+  uint8_t buf[] = {0x42};  // value = 66
+
+  for (auto _ : state) {
+    auto result = ProtoVarInt::parse(buf, sizeof(buf));
+    benchmark::DoNotOptimize(result);
+  }
+}
+BENCHMARK(BM_ProtoVarInt_Parse_SingleByte);
+
+static void BM_ProtoVarInt_Parse_TwoByte(benchmark::State &state) {
+  // Two-byte varint (128-16383)
+  uint8_t buf[] = {0x80, 0x01};  // value = 128
+
+  for (auto _ : state) {
+    auto result = ProtoVarInt::parse(buf, sizeof(buf));
+    benchmark::DoNotOptimize(result);
+  }
+}
+BENCHMARK(BM_ProtoVarInt_Parse_TwoByte);
+
+static void BM_ProtoVarInt_Parse_FiveByte(benchmark::State &state) {
+  // Five-byte varint (max uint32 = 4294967295)
+  uint8_t buf[] = {0xFF, 0xFF, 0xFF, 0xFF, 0x0F};
+
+  for (auto _ : state) {
+    auto result = ProtoVarInt::parse(buf, sizeof(buf));
+    benchmark::DoNotOptimize(result);
+  }
+}
+BENCHMARK(BM_ProtoVarInt_Parse_FiveByte);
+
+// --- Varint encoding benchmarks ---
+
+static void BM_Encode_Varint_Small(benchmark::State &state) {
+  // Value < 128 — single byte fast path
+  APIBuffer buffer;
+  buffer.resize(16);
+
+  for (auto _ : state) {
+    ProtoWriteBuffer writer(&buffer, 0);
+    writer.encode_varint_raw(42);
+    benchmark::DoNotOptimize(buffer.data());
+  }
+}
+BENCHMARK(BM_Encode_Varint_Small);
+
+static void BM_Encode_Varint_Large(benchmark::State &state) {
+  // Value > 128 — multi-byte slow path
+  APIBuffer buffer;
+  buffer.resize(16);
+
+  for (auto _ : state) {
+    ProtoWriteBuffer writer(&buffer, 0);
+    writer.encode_varint_raw(300);
+    benchmark::DoNotOptimize(buffer.data());
+  }
+}
+BENCHMARK(BM_Encode_Varint_Large);
+
+static void BM_Encode_Varint_MaxUint32(benchmark::State &state) {
+  APIBuffer buffer;
+  buffer.resize(16);
+
+  for (auto _ : state) {
+    ProtoWriteBuffer writer(&buffer, 0);
+    writer.encode_varint_raw(0xFFFFFFFF);
+    benchmark::DoNotOptimize(buffer.data());
+  }
+}
+BENCHMARK(BM_Encode_Varint_MaxUint32);
+
+// --- ProtoSize::varint() benchmarks ---
+
+static void BM_ProtoSize_Varint_Small(benchmark::State &state) {
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(ProtoSize::varint(42));
+  }
+}
+BENCHMARK(BM_ProtoSize_Varint_Small);
+
+static void BM_ProtoSize_Varint_Large(benchmark::State &state) {
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(ProtoSize::varint(0xFFFFFFFF));
+  }
+}
+BENCHMARK(BM_ProtoSize_Varint_Large);
+
+}  // namespace esphome::api::benchmarks