fix(proof): stop rejecting ffmpeg builds that do have libx264

The libx264 preflight piped `ffmpeg -encoders` straight into `grep -q`. grep
exits at the first match, ffmpeg takes a SIGPIPE, and `set -o pipefail` turns
that into a failed check. Whether it trips depends only on how much ffmpeg
manages to write before grep leaves, so the check rejected precisely the
ffmpeg builds that satisfy the requirement.

This was not a rare configuration. `nix develop`, which flake.nix describes as
the "Sigil reproducible Linux host proof environment", ships ffmpeg-headless
with 223 encoders including libx264 and libx264rgb -- and the proof failed
there every time with "ffmpeg does not provide the required libx264 encoder".
So loopback-proof.sh could not run on Linux in the project's own sanctioned
proof shell. It passed on macOS only because that ffmpeg's shorter output won
the race with grep.

Capture the encoder list first, then match it. Verified on the Linux host: the
three-viewer proof and the focus-handoff proof both now reach
loopback_proof=ok with neutral_before_successor=ok.

commit

1fe6c4b7a1c1825bde0373c856ce451bf54a956b

parent

21a5aeb2599905ada964fafd31b60314436afc98

author

Dami <dami.etoile@gmail.com>