chore(demo): automate VHS recording for demo GIF and README screenshots

[domcyrus]

Summary

• Adds scripts/record-rustnet-demo.sh which builds the release binary, validates the sudo password (3 retries), wires SUDO_ASKPASS so the in-tape sudo -A runs non-interactively, spawns a background traffic generator (curl, dig, ping) so the table is busy during the take, runs both tapes, and optimizes the GIF with gifsicle.
• Extends demo.tape with filter (/443) and process-grouping (a + Space) beats, alongside the existing Overview, Details, and Graph flow.
• Adds screenshots.tape, a slower-paced 1600x1000 tape that emits per-tab PNGs (overview, filter, grouping, details, graph, interfaces) for the README.
• Refreshes assets/rustnet.gif from a fresh VHS render at 1200x720 (2.4 MB after gifsicle).
• Adds a Screenshots section to README.md with the four key tabs in a 2x2 grid.

Test plan

• `bash scripts/record-rustnet-demo.sh` runs end-to-end with only the sudo password as input
• `assets/rustnet.gif` renders cleanly in GitHub
• All four PNGs render in the README's Screenshots section
• Wrong password retries up to 3 times and aborts cleanly if all fail