Add runnable demo fixtures for the shipped UI surface

[blackopsrepl]

Closes #18.

Summary

• add a full-surface demo fixture that exercises the main shipped components together
• add a smaller rail-focused fixture for layout and scheduling validation
• document the demo fixtures and add make demo-serve for local troubleshooting

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Rebuild bundled assets before serving demo fixtures

Make demo-serve depend on assets. The new demos load ../static/sf/sf.css and ../static/sf/sf.js, but this target only starts python3 -m http.server. After make clean (which deletes both bundles in Makefile:228-231) the documented make demo-serve flow serves pages with 404s for the UI assets, and even without clean it can silently validate stale bundles after edits under css-src/ or js-src/ instead of the current sources.

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