sha256Hex
public static String sha256Hex(List<PackFileIndex.PackFile> files, BooleanSupplier cancellationRequested)
SHA-256 over relativePath, 0x00, content, 0x00 for every file in order — identical to the serial digest loop it replaces, but with reads overlapped across a bounded pool.
Return
the digest as lowercase hex
Throws
java.util.concurrent.CancellationException
if cancellation is requested mid-digest