Bug report: Gzip operation produces invalid streams when using comments with header checksum

[williballenthin]

Describe the bug
When the Gzip operation is configured with both a comment and "Include file checksum" enabled, the resulting stream has a corrupted header CRC and cannot be decompressed.

src/core/operations/Gzip.mjs, run() method, lines 65-75

if (comment.length) {
    options.flags.comment = true;
    options.comment = comment;
}
const gzipObj = new Zlib.Gzip(new Uint8Array(input), options);
const compressed = new Uint8Array(gzipObj.compress());
if (options.flags.comment && !(compressed[3] & 0x10)) {
    compressed[3] |= 0x10;
}
return compressed.buffer;

The code manually sets the FCOMMENT flag bit (0x10) after zlibjs has already calculated and written the header CRC16. This post-hoc bit-flip corrupts the checksum.

To Reproduce
add Gzip with Comment = test and "Include file checksum" enabled, input hello hello hello, then attempt to decompress with Gunzip or gzip -d. Result: Error: invalid header crc16.

Screenshots

Additional context
The zlibjs library expects the comment flag via options.flags.fcomment before calling .compress(), which lets it include the flag when calculating the header CRC. Fix:

if (comment.length) {
    options.flags.comment = true;
    options.flags.fcomment = true;
    options.comment = comment;
}
const gzipObj = new Zlib.Gzip(new Uint8Array(input), options);
const compressed = new Uint8Array(gzipObj.compress());
return compressed.buffer;

The manual bit manipulation should be removed entirely — zlibjs handles flag bits internally when configured correctly.

[GCHQDeveloper581]

Link for example to reproduce above:
https://gchq.github.io/CyberChef/#recipe=Gzip('Dynamic%20Huffman%20Coding','','test',true)Gunzip()&input=SGVsbG8gd29ybGQh&oeol=FF