The browser tests flake under full-suite load only; every failing test
passes alone. BOOTH_TRACE=1 keeps a full trace (screenshots and DOM
snapshots) for each browser test that fails. BOOTH_TRACE=light keeps
actions and network only, because the full mode perturbs the timing it
watches: 0/8 red traced against 1/8 untraced on the same tree. Off by
default. Positive control: a deliberately failing test keeps a trace,
and a passing one keeps nothing.
