* web: refactor 2/7 federation
* web: close federation coverage gaps and apply review nits
Address Codecov patch coverage feedback by adding rspec examples for
the 51 lines flagged across the new federation shards (announce,
crawl, validation, http_client, self_instance, instance_metrics,
announcer_threads, lifecycle, signature). Per-shard line coverage in
the federation directory is now 100%.
Apply two review-comment changes: rename the awkwardly-named
http_client_get.rb to instance_fetcher.rb (matching its semantic
role rather than the HTTP verb), and declare PotatoMesh::App::Federation
explicitly in the federation.rb manifest so the namespace is owned by
this file rather than implicitly created by whichever shard happens to
load first.
l5y