Function derive_store_key 

pub fn derive_store_key(
    passphrase: &str,
    salt: &[u8],
) -> Result<Vec<u8>, SyncError>

Derives the store encryption key from a passphrase and a salt.

The salt is supplied by the caller (read from the store’s meta/salt blob), keeping salt I/O in the git layer and key math here. The same passphrase and salt always yield the same 32-byte key.