Expose replay error classification and variant data in FFI

[chavic]

SenderPersistedError / SenderReplayError (and receiver equivalents) lose classification at the FFI boundary. Clients cannot distinguish storage failure vs API failure vs transient error.

Todo

• Define a typed FFI enum for persistence/replay errors.
• Preserve variant classification (storage, API, transient, fatal).
• Add tests that assert classification and recommended handling.
• Update docs/comments to describe recovery semantics

[spacebear21]

Closing per #1264 (comment)