Expose typed error variants for receiver errors in FFI bindings #1270

Receiver‑side errors like PayloadError, RequestError, and SessionError are not exported as typed FFI enums. Callers can’t branch on error causes, which blocks recovery logic in clients and reduces testability.

Todo:

• Export PayloadError, RequestError, SessionError to FFI.
• Preserve variant details (e.g., PSBT parse vs fee mismatch vs prev‑txout mismatch).
• Add unit tests that assert specific error variants in JS/Python/Dart.
• Update integration tests to assert typed errors on bad inputs/headers.
• Document expected error mappings in FFI comments.