MonitorStream::processCommand computed buffer_level by dividing by
temp_image_buffer_count (and via MOD_ADD modulo) while only guarding on
playback_buffer. processCommand runs on the command_processor thread,
started in runStream before temp_image_buffer_count is assigned from
playback_buffer. In that window temp_image_buffer_count is still 0 while
playback_buffer is already > 0, so a command arriving then divided by
zero and raised SIGFPE, crashing nph-zms.
Extract the percentage math into MonitorStreamBufferLevel which returns 0
when the buffer count is not positive, and add Catch2 coverage for the
zero-count and wrap-around cases.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
