Add demo recording tools (asciinema, vhs) and release tooling
(cargo-dist) to nativeBuildInputs. Include a cargo-dist wrapper so
`cargo dist` works as a subcommand. Add scripts/record-demo.sh for
consistent asciinema cast recording at 100x30.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
