[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_decode.cpp

@@ -0,0 +1,119 @@
+#include <benchmark/benchmark.h>
+
+#include "esphome/components/api/api_pb2.h"
+#include "esphome/components/api/api_buffer.h"
+
+namespace esphome::api::benchmarks {
+
+// --- HelloRequest decode (string + varint fields) ---
+
+static void BM_Decode_HelloRequest(benchmark::State &state) {
+  // Manually encoded HelloRequest:
+  // field 1 (string): "aioesphomeapi"
+  // field 2 (varint): 1  (api_version_major)
+  // field 3 (varint): 10 (api_version_minor)
+  uint8_t encoded[] = {
+      0x0A, 0x0D,                                                         // field 1, length 13
+      'a',  'i',  'o', 'e', 's', 'p', 'h', 'o', 'm', 'e', 'a', 'p', 'i',  // "aioesphomeapi"
+      0x10, 0x01,                                                         // field 2, value 1
+      0x18, 0x0A,                                                         // field 3, value 10
+  };
+
+  for (auto _ : state) {
+    HelloRequest msg;
+    msg.decode(encoded, sizeof(encoded));
+    benchmark::DoNotOptimize(msg.api_version_major);
+  }
+}
+BENCHMARK(BM_Decode_HelloRequest);
+
+// --- SwitchCommandRequest decode (simple command) ---
+
+static void BM_Decode_SwitchCommandRequest(benchmark::State &state) {
+  // field 1 (fixed32): key = 0x12345678
+  // field 2 (varint): state = true
+  uint8_t encoded[] = {
+      0x0D, 0x78, 0x56, 0x34, 0x12,  // field 1, fixed32
+      0x10, 0x01,                    // field 2, varint true
+  };
+
+  for (auto _ : state) {
+    SwitchCommandRequest msg;
+    msg.decode(encoded, sizeof(encoded));
+    benchmark::DoNotOptimize(msg.state);
+  }
+}
+BENCHMARK(BM_Decode_SwitchCommandRequest);
+
+// --- LightCommandRequest decode (complex command with many fields) ---
+
+static void BM_Decode_LightCommandRequest(benchmark::State &state) {
+  uint8_t encoded[] = {
+      // field 1: key (fixed32) = 0x11223344
+      0x0D,
+      0x44,
+      0x33,
+      0x22,
+      0x11,
+      // field 2: has_state (varint) = true
+      0x10,
+      0x01,
+      // field 3: state (varint) = true
+      0x18,
+      0x01,
+      // field 4: has_brightness (varint) = true
+      0x20,
+      0x01,
+      // field 5: brightness (fixed32/float) = 0.8
+      0x2D,
+      0xCD,
+      0xCC,
+      0x4C,
+      0x3F,
+      // field 9: has_rgb (varint) = true
+      0x48,
+      0x01,
+      // field 10: red (fixed32/float) = 1.0
+      0x55,
+      0x00,
+      0x00,
+      0x80,
+      0x3F,
+      // field 11: green (fixed32/float) = 0.5
+      0x5D,
+      0x00,
+      0x00,
+      0x00,
+      0x3F,
+      // field 12: blue (fixed32/float) = 0.2
+      0x65,
+      0xCD,
+      0xCC,
+      0x4C,
+      0x3E,
+      // field 20: has_effect (varint) = true
+      0xA0,
+      0x01,
+      0x01,
+      // field 21: effect (string) = "rainbow"
+      0xAA,
+      0x01,
+      0x07,
+      'r',
+      'a',
+      'i',
+      'n',
+      'b',
+      'o',
+      'w',
+  };
+
+  for (auto _ : state) {
+    LightCommandRequest msg;
+    msg.decode(encoded, sizeof(encoded));
+    benchmark::DoNotOptimize(msg.brightness);
+  }
+}
+BENCHMARK(BM_Decode_LightCommandRequest);
+
+}  // namespace esphome::api::benchmarks