The merge queue's "red alone" mark on a pull clears when main moves, so a green pull rejoins without a new head

Part of #5930 (steward, 18:13Z: #5666 sat 36 h green and approved, kept out by the in-memory red mark until its head changed).

Done:

[ ] line.reds in src/checks/queue.ts records the base the pull went red on, beside the head.
[ ] On enqueue, a mark whose base is not the current base is dropped: the pull gates again as a fresh entry, not as a recheck.
[ ] A mark on the same head and the same base still keeps the pull out, as today.
[ ] Tests: red alone, then main moves, then enqueue gates it; red alone with main unmoved still holds it.

Files: src/checks/queue.ts, a test under test/ for the queue.

Before pushing: bun x tsc --noEmit and the queue tests. Pass now and oids in; no wall clock.

The merge queue's "red alone" mark on a pull clears when main moves, so a green pull rejoins without a new head #6493