feat(typescript): expose FLATFILES on the napi-rs client

[userFRM]

Problem

sdks/typescript/src/lib.rs and historical_methods.rs have no FLATFILES bindings.

Proposed solution

Add tdx.flatfileRequest, tdx.flatfileRequestDecoded, tdx.flatfileRequestRaw, plus the 9 convenience methods (tdx.flatfileOptionOpenInterest, etc.). Expose SecType / ReqType / FlatFileFormat as TypeScript enums via #[napi]. Update the generated .d.ts and the npm-package README quick-start.

Impact

Node.js consumers (browser-adjacent tooling, ops scripts) get parity with the Rust core.

[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.