The previous #runSerialized waited on the shared #dispatchTail, then ran
unconditionally: when two or more events queued behind an in-flight run,
each resumed from the same settled await and started its own run in
parallel, defeating the ordering guarantee for a burst landing in one
coalescing window (message_end + agent_end behind a suspended flush).
Each waiter now chains its own link onto the current tail
(tail.then(run, run)), so queued runs start strictly one after another;
the idle path still runs synchronously, preserving the flush timing the
coalescing tests assert on. The in-flight flag clears only when the
settling link is still the tail, so a later chained link's settle does
not clear it early.
Regression test: two message_end events queued behind a suspended window
flush stay serialized (init call count steps 1 -> 2 -> 3 as each gate
opens); fails on the previous implementation.
Slava Zavadsky
326fb24367