Promo v2: live line example

Approved outcome: teach developers by changing a chart, reading its actual source, and running that example outside the monorepo. Deliver one complete line example first; histogram and scatter follow separately.

Keep the Astro promo shell and introduce a FoldKit island at /examples/line/. The island owns interpolation (smooth/linear/step), five values (0–100), and Y-axis domain maximum (100–200). Reset restores the initial settings. Its model is the only source of truth; browser effects are Commands.

Display actual maintained chart/application/CSS files and a settings module that tracks the live controls. Provide copy with success/failure feedback, project ZIP download and Open in StackBlitz. Both exports capture the current settings and use the same maintained files as the preview. Include compiled Foldkit Viz modules from this checkout because published versions lag behind. Use a small Vite project with FoldKit for standalone execution; Astro remains the promo host. No code editor or arbitrary code runner is added in this slice.

All actions and controls are keyboard accessible; preserve light/dark themes, mobile fit, illustrative data labels and a static no-JavaScript explanation. Do not publish or deploy. Preserve the existing uncommitted v1 work.

Verify state/geometry contracts and execute exported project code from a temporary directory. Build the standalone project without workspace imports. Inspect both themes, mobile, source synchronisation, reset, copy/download and playground launch in the browser. Run root check/typecheck/workspace tests/build sequentially where shared build outputs are involved.