Confirms single-frame stepping is reliable: monotonic frame counter
across steps, and the synthetic clip's 1-frame flash event lands on
exactly one frame with clean neighbors. Also documents/works around
three environment quirks hit along the way: locally-built libmpv not
in ldconfig's cache, QApplication resetting LC_NUMERIC and breaking
libmpv's locale requirement, and the playback-restart event not firing
reliably after frame-step (switched to polling estimated_frame_number).
ThePendulum