Artifacts tab: make long version lists

Seen while testing Task 6712's resizable list (18 artifacts, 17 of them report.html): 1. Group superseded versions under their current row ("16 earlier versions", collapsed by default) so the list leads with what's live. 2. Show that the capped list scrolls (a bottom fade or a "+10 more" count): on overlay-scrollbar systems nothing says rows are hidden. 3. Superseded report.html rows have the same name; show each row's version number or exact time so you can tell them apart without hovering. 4. Let ↑/↓ move the selection inside the list, so you can step through report versions without the mouse. Out of scope: the divider and list cap themselves (Task 6712); how artifacts are stored and served. Done means: each item above is in, or dropped with a stated reason; the Artifacts tab is checked at desktop and phone width with a task that has 15+ artifacts; `./dc check full` is green.

Artifacts tab: earlier versions fold under the live row, vN numbering, fade + "+N more", ↑/↓ stepping. Visual check done at pane, desktop and phone with 20 artifacts. PR #5667 merged via lane (full gate). Follow-up draft 6788.