Production showed multi-second windows where every response — routes,
RSC payloads, static assets — waited together while handler durations
stayed under half a second: the process stalled, and nothing
route-shaped could name it. A self-rescheduling tick now records how
late it fires; a tick past the stall threshold emits its own
event_loop_stall background event, and every wide event carries the
recent worst lag so the first request finishing after a stall is
annotated with it. Measured via a timeout chain rather than
perf_hooks.monitorEventLoopDelay — the histogram's sampling timer races
a sibling read-and-reset interval in the same timers phase and the
stall sample can vanish into the reset window.
