[codex] fix FAK order type serialization

[SebastianBoehler]

Summary

• Serialize OrderType::FAK as FAK instead of the unsupported IOC wire value.
• Clarify the public enum comment and add a regression test for order-type serialization.
• Bump the package version to 1.3.0 and update README release references.

Validation

• cmake -S . -B /tmp/polymarket-issue22-localdeps-build ... && cmake --build /tmp/polymarket-issue22-localdeps-build --target test_order_type_serialization test_order_execution -j 8
• ctest --test-dir /tmp/polymarket-issue22-localdeps-build --output-on-failure -E test_oracle_watcher_historical

Full ctest was also attempted. The only failure was test_oracle_watcher_historical, which hit HTTP 403 from https://polygon-bor.publicnode.com.