Year chips on the releases heatmap. The default view stays the rolling
last-12-months band (today anchored at the right edge — the graph never goes stale
and has no dead future space). New chips beside the summary — "Last year" plus one
per calendar year with releases — switch to fixed Jan→Dec views of that year,
exactly GitHub's default-vs-drill-down model. The current year keeps its full
Jan→Dec month axis with the not-yet-elapsed part rendered empty.
lib/changelog.ts: the grid walker is now shared; new buildYearCalendar(entries, year) (Sunday-aligned calendar-year window, out-of-year + future days as filler)
and listReleaseYears(). Year views are computed client-side on demand from the
per-day data the page already ships; SSR still renders the rolling view, so there's
no hydration mismatch.
