New package mirroring `kernelcat`: catalog + SHA256-verified HTTP
fetch of `.tar.zst` bundles that contain rootfs.ext4 + manifest.json.
Mounted empty (version:1, entries:[]) so nothing is pullable via the
bundle path yet; wiring into `banger image pull` lands in a later
phase.
- catalog.go: Catalog/CatEntry, LoadEmbedded, ParseCatalog, Lookup,
ValidateName.
- fetch.go: Fetch(ctx, client, destDir, entry) downloads the bundle,
verifies sha256, extracts exactly rootfs.ext4 and manifest.json
into destDir, returns the parsed manifest. Rejects unexpected tar
entries, unsafe paths, non-regular files, and cleans up partial
writes on failure.
- Thirteen unit tests (happy path + every failure mode).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
