Function encrypt_tombstones 

pub fn encrypt_tombstones(
    tombstones: &[Tombstone],
    key: &[u8],
) -> Result<Vec<u8>, SyncError>

Serializes and encrypts the tombstone set into a git-blob-ready buffer.

Stored at meta/tombstones in the store tree so deletions of child records propagate across machines alongside the session blobs. Uses the identical serde_json -> gzip -> encrypt_data pipeline as encrypt_session_record.