[mparrett]

The site root was the mandelbrot demo, microgpt sat at /microgpt/, and pathtrace and ptcanvas weren't deployed. This adds an index page with a screenshot tile per demo and deploys every browser demo at its own path.

Site layout

• / is a static index (site/index.html): a large mandelbrot tile, then pathtrace, ptcanvas, microgpt, mandelbrot-aot (native, links to its source), and a short "run it locally / source" tile. It's a dark grid that collapses to one column on phones.
• /mandelbrot/, /pathtrace/, /ptcanvas/ and /microgpt/ each hold the single-file demo and their own coi-serviceworker.js.
• Old root links with a query (?x=…&zoom=…) redirect to mandelbrot/, so existing start-view links keep working. A bare root link now opens the index.
• The root keeps a copy of coi-serviceworker.js, so browsers that registered it when mandelbrot lived there keep a valid script.

Screenshots (site/img/, about 480 KB in total) were captured from these builds with let-go v1.13.0 in headless Chromium:

• mandelbrot at ?x=-0.743643&y=0.131825&zoom=250&maxiter=384;
• pathtrace and ptcanvas after about 8 minutes of refinement (16 and 21 samples/pixel, so still slightly grainy);
• the microgpt page;
• a frame from the existing mandelbrot-aot-zoom.gif.

Checks

• New test/pages_site_test.py serves _site header-free under /let-go-lab/. It checks that the index's images load, its local links resolve, it doesn't overflow at 1280 or 390px, and the redirect works. It also checks that every demo becomes cross-origin isolated through its own service worker and boots without page errors.
• pages.yml runs that test before deploying, and now also runs the build and the test on pull requests, without deploying them.
• Locally (v1.13.0 build): all 7 checks pass. bash -n passes on build-pages.sh; shellcheck isn't installed here.

Adding a browser demo now means adding it to DEMOS in build-pages.sh and test/pages_site_test.py, plus a tile in site/index.html (noted in CLAUDE.md).

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