A crash mid-command (e.g. the nvbandwidth reboot) previously lost that
job's entire output: streamExecOutput only buffered stdout/stderr in
memory and the job's log file was written once, after the process
exited. It now also streams each line straight to that file as it
arrives, so whatever printed before a crash survives.
Root filesystem here is a tmpfs overlay (toram boot), so the only real
persistence boundary is blackbox's mirror to removable media, not the
local write itself. platform.SetJobBoundaryHook lets app wire a touch
of a small kick-file after each job's output is written; blackboxWorker
now polls that file's mtime alongside its normal adaptive timer and
syncs immediately on a kick instead of waiting out the current flush
period (up to 30s).
