ctxCard takes opts.foldable (Routing passes it; the Usage page's cards
don't): its title becomes a button with a chevron, aria-expanded, and a
click toggles .shut on the card, which hides all but the head. The choice
is kept in localStorage (magpie.ctxShut). The head is morphed in place, so
the click reads the button from the event, and the card that is there
keeps .shut across live patches; a new card takes it from the saved choice.
Verified: routing-ctx-fold.test.cjs (new), at 420 and 1100 wide in
Chromium and WebKit, 3/3 each: open, then folded to one line (<60px) with
the figure, the list under it up by >200px, nothing runs over at 420px;
another request picked is still folded; after a reload still folded;
opened again, 400 cells and the same height as before. It fails on the old
code (no .ctx-fold). routing-*, context-*, usage*, group-context,
click-scroll, panel-routing, old-webkit, gui-ja/de/zh-tw: WebKit 371/371;
Chromium 370/371 once at --test-concurrency=4, routing-blocked's "picking
the request moved the page", which passes 3/3 alone and in two more
routing-* runs at the same concurrency (196/196 each). Not filed as caused
by this change; seen once under load.