ExportLegacy now skips assets with non-empty Variants (installer,
rocm, fxdependent, etc.) and formats Node.js doesn't handle (.deb,
.rpm, .snap, .appx, .tar.zst, .tar.bz2, .7z). This ensures the
_cache/ JSON files are compatible with the legacy Node.js server.
Also fix test data to use dotted format strings (.tar.gz) matching
what the classifier actually produces.
Drop VariantTagger interface and map-based lookup. Each per-installer
package now exports a plain TagVariants function. webicached dispatches
via a switch on package name, consistent with fetchRaw and
classifyPackage.
Move variant detection logic from inline functions in webicached to
per-installer packages (internal/releases/{bun,fish,git,lsd,node,
ollama,pwsh,xcaddy}). Each exports a Tagger implementing the new
storage.VariantTagger interface. webicached uses an explicit map
of package name → tagger, no magic registration.
Extra is for version-related sort metadata (build numbers, etc.).
Variants captures build qualifiers like "rocm", "jetpack5",
"fxdependent", "installer" — things the resolver should skip by
default unless explicitly requested.
Also update format classification docs: most formats (.pkg, .deb,
.dmg, .msi) are extractable — only .exe is ambiguous and needs
the "installer" variant tag when it's not the actual binary.
storage.Store is the read/write interface for release asset storage.
storage.Asset uses correct terminology (Filename, Format) internally.
storage.LegacyAsset / LegacyCache preserve the Node.js wire format
("releases", "name", "ext") for backward compatibility.
fsstore writes to _cache/YYYY-MM/{pkg}.json with atomic rename,
matching the existing Node.js layout. The Node.js server can read
files written by Go and vice versa.
