multi-seg, multi-path: Downloading sequences in zip with files for each segment

[anna-parker]

Downloading all nucleotide sequences (one file per segment in a zip) would also be cool)

[claude]

This issue may be related to #4351 ("Support streaming fasta containing sequences of all segments"), which also requests multi-segment sequence downloads in a single operation. The approaches differ — #4351 proposes a single combined FASTA, while this issue requests a zip with separate files per segment — but they address the same underlying user need. Worth coordinating these feature requests.