nllb model archive is XZ but named .tar.bz2; installer picks decompressor by extension and fails

[staging-devin-ai-integration]

Summary

The marketplace nllb model archive is named …-int8.tar.bz2 but is actually XZ-compressed. The installer picks its decompressor purely from the filename extension (model_archive_kind, apps/skit/src/marketplace_installer.rs:1905), so it routes the XZ stream into BzDecoder and fails with Failed to read archive entry. The download itself is intact (sha256 matches the manifest).

There are really two issues here:

1. Registry/packaging: the archive is mislabeled (.tar.bz2 for XZ content).
2. Installer robustness: model_archive_kind trusts the extension instead of sniffing the actual compression.

Details

• model_archive_kind (apps/skit/src/marketplace_installer.rs:1905-1944) maps .tar.bz2 → ModelArchiveKind::TarBz2, which is decoded with bzip2::read::BzDecoder (marketplace_installer.rs:1642).
• The nllb archive's magic bytes are XZ (FD 37 7A 58 5A 00), not bzip2 (BZh), so the decoder errors out.
• Manually re-extracting with xz and pointing the plugin at the result works, confirming the content is fine.

Repro

1. Install the nllb plugin/model via the marketplace.
2. Observe Failed to read archive entry during extraction.
3. file <archive> → reports XZ compressed data despite the .tar.bz2 name.

Suggested fix

• Primary: repackage/rename the nllb model so the extension matches the actual compression (.tar.xz for XZ content, or recompress as real bzip2).
• Hardening (optional, defense-in-depth): make model_archive_kind sniff the leading magic bytes and fall back to / override the extension-based guess, so a mislabeled archive can't silently break installs. A small unit test over the magic-byte → kind mapping would catch regressions.

Context

Found while adding CI coverage for the official oneshot samples (#542). No server code was changed in that PR; filing the packaging + optional installer-hardening work here.