- file.c: File/FileMetadata lifecycle and local disk helpers (~110 lines)
- file_send.c: client-side send path (file_send_single_calls, file_send_sendfile)
- file_receive.c: server-side receive/save path (file_receive, receive_incremental_check,
receive_manifest, file_save_to_disk)
- file_types.h holds shared struct definitions; file.h remains an umbrella header
so existing includes are unaffected
Completes the transfer/protocol separation started in PR #212
