Add ruby_event_store-browser-swimlane extension gem

[pjurewicz]

Summary

A browser extension gem (contrib/ruby_event_store-browser-swimlane) comparing several streams side by side: their events merged into one newest-first timeline table, one column per stream, with infinite scroll and a created-at/valid-at sort toggle. An event linked into several compared streams renders once, as a link in each matching column.

• Data layer: each page is composed from existing single-stream reads — one time-ordered (as_at/as_of) read per compared stream, k-way merged by event time. Count is the read depth per stream, not the page size: a page emits everything above the completeness horizon (the highest chunk end among the streams whose read was cut), so nothing provably complete is thrown away and a page carries between count and count × streams events. The shared cursor is that horizon timestamp; a chunk cut exactly at it has the rest of that timestamp's group drained with between(t..t), so the next page asks for strictly older events and nothing is dropped or repeated. A mass of identical timestamps renders as one long page.
• Built entirely on the extension API: /compare and /compare/more via register_routes (json + render_partial), an entry link on every stream page via stream_links, its own Stimulus controllers via scripts (a separate Stimulus application importing the vendored stimulus module; timezone display synced with the core selector through the shared localStorage key). No core changes.
• CI wired via support/ci/generate (test / mutate / coverage workflows).

Relies on #1954 (in-memory repository: limit after time filters) and #1950 (Sequel: time sort on backward reads) — both already merged.

Supersedes the in-repo approach of #1949.

Test plan

• 36 specs: reader unit coverage (merge, cursors, timestamp groups, linked events, both sort axes) and web coverage through Browser::App.for(extensions:) behind Rack::Lint
• Mutation testing — 100% (428 mutations)