Docs refresh + open-source clean-clone runnability (250 genres, package.json)

DOCS (task: "update docs and info in the app"):

• Genre count corrected everywhere: 228/240/249 -> 250 (matrix prints 250/250, verified). index.html about panel, how.html, README.md, CLAUDE.md, CONTRIBUTING.md.

• index.html help line already reflects the four views (map/viz/video/aliens) + the duration control.

• Added a root package.json declaring the ONE browser-test dep (playwright ^1.57.0), and gitignored /node_modules/.

• CONTRIBUTING documents the browser-gate setup.

Verified: verify.sh (matrix/validate/engine/prove/matproof/poscover/coordscover) all PASS at 250/250; full-boot green. No app behaviour change.

▶ read how a song is made — the drawn pipeline itself https://stellate.app/how.html

8dcadb7 https://github.com/aboard-io/stellate/commit/8dcadb7b45ed60dd8ad90b15ac4f976541f33711