swimline view

[pjurewicz]

Summary

• Adds a "compare streams" view to ruby_event_store-browser: from a stream's page you can add other streams and see their events merged into one newest-first timeline table, one column per stream, with infinite scroll. An event linked into several compared streams renders once, as a link in each matching column.
• The data layer lives entirely in the browser gem and composes each page from existing single-stream reads — no new repository/core API. Each compared stream is read in as_at order and the per-stream pages are k-way merged by event time, so the merge inputs are sorted by the merge key and the result is correct also for linked events and custom timestamps.
• The timeline can be sorted along either axis of the bi-temporal model: created-at (as_at, default) or valid-at (as_of), toggled in the header. The whole pipeline — per-stream reads, merge key, cursor comparisons, group draining and the displayed time column — follows the chosen axis.
• Pagination uses a plain timestamp cursor. Pages never split a group of events sharing one timestamp: a page runs to the end of its last timestamp (fetching the rest of the group with between(t..t) when the per-stream page got cut exactly there), so the next page asks for strictly older events and nothing is dropped or repeated. Pages can therefore be a little longer than the nominal count — and a mass of identical timestamps (e.g. a backfill with second precision) renders as one long page.
• Fixes the in-memory repository applying limit before the time filters, unlike the SQL adapters where WHERE precedes LIMIT — limit + older_than is exactly the combination this pagination relies on. A shared lint example pins the ordering for all adapters.

Test plan

• ruby_event_store suite (incl. the new limit-vs-time-filter lint example) — green
• ruby_event_store-active_record, contrib/ruby_event_store-sequel, contrib/ruby_event_store-rom suites — green (no adapter changes; lint example passes everywhere)
• ruby_event_store-browser suite incl. compare view, JSON endpoint, UI/Capybara — green
• Mutation testing at 100% for the browser gem diff and the in-memory fix

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