chore(ci): add benchmarks and test matrix

[SebastianBoehler]

Summary

• add optional benchmark targets for V2 signing, V2 order payload serialization, local HTTP fixture warm/cold requests, and WebSocket JSON parse throughput
• document benchmark build/run commands and how to interpret results without CI timing thresholds
• update CI to run Debug and Release builds on Linux and macOS
• enable examples, tests, and benchmark target builds in CI, and run ctest
• keep live smoke disabled by default behind the POLYMARKET_RUN_LIVE_SMOKE repository variable

Validation

• cmake -S . -B /tmp/polymarket-cpp-client-verify -DPOLYMARKET_CLIENT_BUILD_EXAMPLES=ON -DPOLYMARKET_CLIENT_BUILD_TESTS=ON -DPOLYMARKET_CLIENT_BUILD_BENCHMARKS=ON
• cmake --build /tmp/polymarket-cpp-client-verify -- -j4
• ctest --test-dir /tmp/polymarket-cpp-client-verify --output-on-failure
• /tmp/polymarket-cpp-client-verify/benchmark_v2_signing 10
• /tmp/polymarket-cpp-client-verify/benchmark_order_payload 10
• /tmp/polymarket-cpp-client-verify/benchmark_http_fixture 5
• /tmp/polymarket-cpp-client-verify/benchmark_ws_parse 10
• git diff --check