Add npm + PyPI download stats as an adoption signal

Official public endpoints - the npm registry downloads API and pypistats.org - publish package download counts keylessly. When a topic resolves to a package (or a GitHub repo already found names one), download trends answer 'is this actually being adopted', which stars alone don't.

Best implemented as an enrichment of GitHub results rather than a standalone search: for top repos, look up the matching package and attach weekly downloads to that context entry.

Constraint check: official public APIs, keyless. ✅

Start by tracing the existing GitHub result flow to identify where top repositories are enriched and where context entries are assembled. Review the npm registry downloads API and pypistats.org endpoints, then define how a repository maps to a package. Done means matching npm or PyPI packages have weekly download trends attached to the relevant GitHub result using keyless public APIs.

Written by the indexing model from the issue text.