Release heatmap is now interactive; compact releases page

Changed

• The /releases heatmap now behaves like GitHub's. Hovering a day shows a tooltip (date · release count · what shipped); clicking or tapping a day pins it into a compact summary panel under the grid — date, count, and the day's release titles — with an explicit View details button that expands the full notes (summaries + links). The latest active day is pre-selected so the panel is informative at a glance, including on mobile where there's no hover.
• The grid renders the full year band, GitHub-style — empty months included — with cells sized (10px pitch) so all 53 columns fit the prose column with no horizontal scrolling on desktop. On narrow screens it scrolls but opens at the most recent activity, and the legend lives outside the scroll area so it's always visible. (buildActivityCalendar also grew maxWeeks/leadWeeks/minWeeks options for data-trimmed windows; the releases page pins minWeeks: 53 for the year view.)
• The full timeline is collapsed by default behind a "Browse the full release history" toggle (components/docs/ReleaseHistory.tsx) — the page stays short; deep links (#2026-07 month anchors from the TOC, or a release slug) auto-expand and scroll.