cmd opens '>>' redirection targets write-exclusive, so when the STARTED
and terminal hook children appended to one shared log concurrently, the
loser of the open race hit a sharing violation and silently dropped its
marker line -- a permanent loss that surfaced as an intermittent
Windows-CI-only failure of the macro-bracketing suite (the engine's exit
drain was verified sound: spawn intent is registered synchronously at
fire time and covers macro-context fires). One file per event makes
writer overlap impossible while keeping every exact-count assertion
byte-identical.