The queue:check sentence says when its checks are held by a red main, not waiting behind the lane #3120

Part of #3116. laneSentence in kernel/vitals.ts said "10 checks wait behind one lane … the lane runs a check for 3 min". All 10 were pull checks held by the red main (checks/red_main.ts unheld), and the lane was free. The sentinel filed that as a lane problem (#3116) next to the real cause (#3105).

Mechanical fix: vitals counts the queued checks a red main holds (heldChecks already does this in one query). When every check past laneWait is held, queue:check isn't raised, because the red: cause already names them. When only some are held, the sentence counts just the free ones and adds "; N more held by a red main". Test in the vitals suite: 10 held pull checks past laneWait and an idle lane raise red: and not queue:check.