Replace the per-package JSON file cache (metadata-v1.4/, metadata-ff-v1.4/)
with a single SQLite database (metadata.db) for registry metadata caching.
Benefits:
- Cheap conditional request header lookups (etag/modified) without parsing
the full metadata JSON — enables If-None-Match/If-Modified-Since with
minimal I/O overhead
- Full metadata can serve abbreviated requests — if a package was previously
fetched as full (e.g., for trustPolicy or resolutionMode), the resolver
reuses it instead of making another registry request
- Eliminates hundreds of individual file read/write/rename operations per
install, replaced by SQLite WAL-mode transactions
- Removes the runLimited/metafileOperationLimits concurrency machinery —
SQLite handles concurrent access natively
New package: @pnpm/cache.metadata — SQLite-backed MetadataCache class
modeled after @pnpm/store.index, with getHeaders() for cheap lookups,
get() with abbreviated→full fallback, and set()/updateCachedAt().
