fix: force gzip flush setting to Z_FULL_FLUSH

[nlf]

i tested a full matrix of combinations for both flush and level (i also did some experimenting with chunkSize but that yielded nothing meaningful), the combinations that resulted in identical packed results across osx/m1, osx/intel, ubuntu/intel, and arch/intel are as follows:

match: { flush: 0, level: 0 }
match: { flush: 1, level: 0 }
match: { flush: 2, level: 0 }
match: { flush: 3, level: 0 }
match: { flush: 3, level: 9 }

since setting the level to 0 is effectively no compression, that felt like a non-starter, so i went with setting flush to Z_FULL_FLUSH. this does yield a bit of a size increase as compared to the default of Z_NO_FLUSH (~15% on my test package which is 5.1mb unpacked), but that feels like a pretty reasonable tradeoff to get a bit more consistency

References

Related to #76