feat(cpp): expose FLATFILES on the C++ SDK

[userFRM]

Problem

sdks/cpp/include/ and sdks/cpp/src/ carry no FLATFILES headers or wrappers.

Proposed solution

Once the FFI layer (#434) lands, add:

• sdks/cpp/include/thetadx_flatfiles.h declaring tdx::FlatFileFormat, tdx::SecType, tdx::ReqType, tdx::FlatFileRow.
• Methods on the existing ThetaDataDx C++ class: flatfile_request, flatfile_request_decoded, flatfile_request_raw, plus 9 convenience methods.
• A sdks/cpp/examples/flatfile_demo.cpp example mirroring crates/thetadatadx/examples/flatfile_demo.rs.
• CMake target wiring in sdks/cpp/CMakeLists.txt.

Impact

C++ users get parity. CMake build covers the new header.

[userFRM]

Closed by PR #514. Flatfiles SDK parity shipped via hand-written wrappers (commits 58deb80, abc265d, 269c16f, c81eea1):

• Python: tdx.flat_files.{quotes,trades,ohlc,...} returning FlatFileRowList with .to_arrow() / .to_polars() / .to_pandas() / .to_list() terminals
• TypeScript: tdx.flatFiles.* with .toArrowIpc() / .toJson() / .len()
• C ABI: tdx_flatfile_request_decoded, tdx_flatfile_rows_to_arrow_ipc, tdx_flatfile_request_to_path
• C++: tdx::FlatFiles namespace + tdx::FlatFileRowList RAII

Built on crates/thetadatadx/src/flatfiles/arrow.rs::rows_to_arrow (dynamic-schema Arrow) — hand-written rather than codegen because flatfiles columns are determined at runtime by (SecType, ReqType), which the static-schema codegen cannot represent. See #439 for that decision.