Add ?demo mode and recording script for marketing clips

[PaulHax]

Summary

Adds an auto-piloted walkthrough activated by appending ?demo to the URL, plus a Playwright-based recording driver. Used to produce a ~22 s 4K demo clip stepping through scenario+baseline → values → side-by-side compare, with view-transition morphs between screens.

The mode is gated entirely on the ?demo query — zero impact on normal users visiting the site.

What's in the commit

• static/demo.js (new, ~30 lines) — drives goToStep / setValue with timed waits, shows the title intro, signals completion.
• static/demo.css (new, ~50 lines) — hides top nav / TOC / Back-Next; flat background; centered headers; bumps view-transition animation-duration to 1.4 s so the morph reads on video.
• record_demo.py (new) — serves static/ on a local port, runs Playwright at logical 1280×600 with DPR=3 (true 4K bitmap), trims first 0.5 s with ffmpeg so the title is on frame 0.
• static/index.html — gates demo.js import on ?demo; sets html.demo + flat background synchronously so the page never flashes the gradient.
• static/guide.js — exposes window.__demo hook (gated on ?demo); adds 3↔5 to the `isMorph` list so Personal Values → Compare animates smoothly.
• .gitignore — excludes `spark-demo-*.{mp4,webm}` outputs and `_video/` temp dir.
• README.md — "Recording a demo clip" section.

Test plan

• `python3 -m http.server -d static 8000` — normal mode still works at `localhost:8000`, no demo hooks visible.
• `localhost:8000/?demo` — auto-walkthrough runs: title (~2.5 s) → Implicit Values → Personal Values → Compare. No TOC/nav/Back-Next visible.
• `pip install playwright && playwright install chromium && python3 record_demo.py` — produces `spark-demo-4k.webm` with title on frame 0.
• `ffmpeg` MP4/HD conversions from the README produce playable files.