A pull the queue holds as red alone says so on its page: "red alone at <time>, waiting for a new head or a new main"

Part of #5930: the red mark must not be invisible.

Done:

[ ] src/checks/queue.ts exports a pure read of a pull's red mark (time, head, base), or undefined.
[ ] The pull page draws one muted line under its check line when the mark is set; nothing when unset (no empty box, no layout shift).
[ ] Text is escaped; the time is a <time> element.
[ ] Test: a marked pull's page has the line; an unmarked one does not.

Files: src/checks/queue.ts, the pull page under src/web/pages/, a test.

Before pushing: bun x tsc --noEmit, the tests you touch, bun scripts/design-lint.ts.

A pull the queue holds as red alone says so on its page: "red alone at <time>, waiting for a new head or a new main" #6494