Build a capped-CRF ABR ladder with FFmpeg and prove it with VMAF (x264, NVENC, SVT-AV1)

TL;DR We'll convert a 4-rung CBR ladder to capped CRF, write a small Python sweep that encodes a set of clips at several CRF and cap values, score each with VMAF, and report average VMAF, lowest-frame VMAF, bitrate, and the share of seconds that hit the cap. Then we'll port the winning settings to NVENC and SVT-AV1. A fixed-bitrate ladder spends the same on every second of every title. Capped CRF lets the encoder spend less on easy content and holds a ceiling on hard content. The catch is that it has two knobs (the CRF value and the cap) and most people only tune one.