When a cron job fires at 13:00:00.014 and completes at 13:00:00.021,
computeNextRunAtMs was flooring nowMs to 13:00:00.000 and asking croner
for the next occurrence from that exact boundary. Croner could return
13:00:00.000 (same second) since it uses >= semantics, causing the job
to be immediately re-triggered hundreds of times.
Fix: Ask croner for the next occurrence starting from the NEXT second
(e.g., 13:00:01.000). This ensures we always skip the current/elapsed
second and correctly return the next day's occurrence.
This also correctly handles the before-match case: if nowMs is
11:59:59.500, we ask from 12:00:00.000, and croner returns today's
12:00:00.000 match.
Added regression tests for the spin loop scenario.
de6cc05e7e