feat(clob): centralize fast order execution path

[SebastianBoehler]

Summary

• move order creation/post/cancel methods into a focused clob_order_execution.cpp module
• add an internal OrderExecutionContext plus shared V2 order payload serialization helpers
• add deterministic tests for limit-order amount math, signer/funder context behavior, and batch payload serialization

Validation

• cmake -S . -B /tmp/polymarket-cpp-client-verify -DPOLYMARKET_CLIENT_BUILD_EXAMPLES=ON -DPOLYMARKET_CLIENT_BUILD_TESTS=ON
• cmake --build /tmp/polymarket-cpp-client-verify -- -j4
• ctest --test-dir /tmp/polymarket-cpp-client-verify --output-on-failure
• git diff --check