Force keyframe-aligned GOPs across an ABR ladder with FFmpeg

TL;DR

If your stream hitches on quality switches but shows zero rebuffering, your ABR renditions probably have keyframes in different places. We'll encode a ladder with identical, forced, closed GOPs in FFmpeg, then verify alignment with ffprobe and MP4Box so it can't regress.

The bug

A clean ABR down-switch needs both renditions to have a keyframe (IDR) at the same media time. If 1080p has a keyframe at 4.0s and 720p's nearest is at 4.3s, the player can't splice cleanly and you get a micro-freeze. Your QoE dashboard says zero rebuffers because, technically, nothing rebuffered. The seam just collided.

Versions: ffmpeg 7.x, MP4Box (GPAC), tested with x264 and NVENC.