bench: add v0.9 benchmark suite

[zuudevs]

Summary

Adds the v0.9 benchmark infrastructure and the five benchmark areas required by the roadmap.

Benchmark infrastructure

• adds opt-in CPP_REQUEST_BUILD_BENCHMARKS (default OFF)
• reuses/pins Google Benchmark v1.9.5 as a build-only development dependency
• disables Google Benchmark's own tests/install path
• adds aggregate cpp_request_benchmarks target
• adds cpp_request_benchmark_smoke target that runs all benchmark executables with a short minimum time

Benchmark coverage

1. request serialization
   • POST request with headers, raw query, appended encoded query parameters, and multiple body sizes
2. response/header parsing
   • HTTP/1.1 response with 24 custom headers and multiple body sizes
3. chunked decoding
   • 4 KiB chunks across 4 KiB, 64 KiB, and 1 MiB decoded payloads
4. end-to-end loopback client overhead
   • local-only steady-state requests with 32-byte and 4 KiB responses
5. connection reuse vs reconnect
   • persistent Client keep-alive path compared directly with a fresh Client/TCP connection per request

Loopback benchmark server

• native local TCP only
• no public network dependency
• supports sequential requests on retained HTTP/1.1 connections
• supports repeated reconnects for comparison benchmarks

CI

Adds a dedicated Release-only benchmark smoke matrix for Ubuntu, Windows, and macOS. Normal six-job build/test CI keeps benchmarks disabled, so ordinary development builds do not pay the Google Benchmark dependency cost.

The benchmark workflow only triggers for benchmark/source/header/CMake changes relevant to the measurements.

Scope

This PR establishes benchmark executables and smoke verification. It does not make performance claims or tune implementation based on unmeasured assumptions; detailed result reporting can be done after the suite is merged and run on stable hardware.