docs: refresh snapshot size/format for zstd + model2vec (snapshot-20260609)

Measured by installing each profile from the published snapshot-20260609
(.tar.zst, 353,295 docs):
- download 1.53 GB (.tar.zst, zstd -9) — replaces the stale "~1.9 GB" gzip figure
- compact 3.1 GB, balanced 5.5 GB, prebuilt 8.6 GB (du)
- resources 1.3 GB incl. the ~125 MB model2vec model (was ~23 MB MiniLM)
- prebuilt MD+HTML: 504 MB + 615 MB content across 706,590 files
- .tar.gz -> .tar.zst format refs across self-hosting, architecture, runbooks,
  perf, security; release-download globs made format-agnostic for old gzip tags.