feat: Support gzip and zstd compressed tiles in the tileset

[kinkard]

closes #1

This PR introduces support for tarballs with gzip- or zstd-compressed (.gph.gz / .gph.zst) tiles. On-disk compression is sniffed from the first tile's filename at startup and assumed uniform across the archive.

Source	Accept-Encoding	Response	Work
Plain	—	plain	raw bytes (decode is a no-op)
Plain	gzip	gzip	encode → gzip
Plain	zstd	zstd	encode → zstd
Plain	gzip, zstd	zstd	encode → zstd (prefer zstd)
Gzip	—	plain	decode
Gzip	gzip	gzip	passthrough
Gzip	zstd	zstd	decode → encode zstd
Gzip	gzip, zstd	gzip	passthrough (beats re-encode)
Zstd	—	plain	decode
Zstd	gzip	gzip	decode → encode gzip
Zstd	zstd	zstd	passthrough
Zstd	gzip, zstd	zstd	passthrough (beats re-encode)

And also adds zstd as an output option (was gzip-only).