Fix asciinema player path, improve VisiData demo timings, clean prompt

- Fix asciinema-player cast file path for use_directory_urls (../ -> ../../)
- Rename "Validating BIDS Files" -> "Validating BIDS Datasets"
- Longer pauses after demo-say narration (2s -> 4s) for readability
- Add pauses after typed VisiData commands (go-col-regex, search)
- Use clean "validation-demo $" prompt instead of leaking absolute paths
- Add record.sh driver for headless asciinema recording via Xvfb
- Re-record visidata-demo.cast with all improvements

Co-Authored-By: Claude Code 2.1.92 / Claude Opus 4.6 <noreply@anthropic.com>

Author: yarikoptic