Compute the Activity rows at build time, with tests #276

Part of #271 (Phase 4).

Why. Every Activity view builds on one set of grouped rows: the /news/ rail, the home strip, the restyled /activity/ and the feed. The round-2 handoff states the display rules as JavaScript (reference/activity-data.js: buildRows, week, range, railRows, byWeek, byMonth, panelSummary) for porting to a Jekyll plugin. Its README is in the round 2 outcome.

Liquid can't do the grouping reliably: it needs a regex and keyed merges. Production (deploy.yml) and the Netlify previews both run a plain bundle exec jekyll build, so plugins run everywhere.

Amendments from the round-2 review, which the port must implement:

• A complete sort order, with no reliance on input order. The prototype breaks ties by input order through JavaScript's stable sort, and Ruby's sort isn't stable.
  • Days run newest first.
  • Within a day: lecture updates, then translations, then releases.
  • Within a type: by the lower-cased project name, then the exact name, then version, then url.
  • A merged translation lists its PRs in edition order.
• Same-day entries for the same series merge into one row, as translations already do. Lecture sites published twice on one day 10 times across 5 sites between June and September 2026.
• A release group lists each project once in its title.
• Empty data returns nothing rather than failing. panelSummary([]) throws in the prototype.

Work.

• _plugins/activity/rows.rb: a pure module, with no Jekyll dependency.
• _plugins/activity_generator.rb: a generator that exposes site.data.activity_view to Liquid. It holds:
  • all rows;
  • the rail: the latest 6 rows of each type, with week groups and an "in All" flag;
  • the log's months and days;
  • the panel summary;
  • the strip rows;
  • the log's first date.
• Tests in .github/scripts/test-activity.rb (not under _plugins/, which Jekyll loads), run against a frozen fixture copy of today's 40 entries. Add the test step to build.yml beside the data check (line 20 on Split updates into News and an Activity feed #265's head, 44f5479).

Acceptance criteria

• On the fixture, the rail shows:
  • All: 6 rows under "Sep 21–27" and "Sep 14–20";
  • Releases: back to jlgametheory v0.2.0 (Aug 16);
  • Lectures: Sep 27 back to Jul 23, under four week headings;
  • Translations: 2 rows.
• On the fixture, the strip rows, the panel summary ("6 updates in September · latest Sep 27") and the log (30 rows: 6/8/11/5) match the handoff.
• Shuffled input gives identical output, and Date-object input gives the same output as string dates.
• Week datetimes use ISO weeks: 2026-12-28 is 2026-W53, and 2029-12-31 is 2030-W01.
• Tests run in CI. No visible change to the site.