test: assert the real app renders instead of a deleted starter skeleton

[PouyanJay]

Fixes #11.

npm test failed on main, so the repo had no working automated check at all. The suite was still asserting the vinext starter's loading skeleton — a app/_sites-preview/ directory, a react-loading-skeleton dependency and a "Starter Project" title — none of which have existed since Anatomy Atelier was built on the template.

What it asserts now

Six tests against what the built worker actually serves:

• serves the document — 200, text/html
• page identity — title, description, og:image, apple touch icon
• organ library — present, and all nine organs in it
• the selected organ is server-rendered, not an empty shell — heart's poetic line, system, scientific name, tissue and a clinical condition all come through SSR
• viewer mount and hotspot text equivalent — three-mount, viewer-tools, and the hotspot-index list. That last one is worth guarding: the hotspot dots live inside the canvas, so this list is the only thing assistive tech can reach.
• no starter residue — the old check is kept but inverted, failing if any trace of the template reappears

Verification

✔ serves the document
✔ renders the page identity and its social metadata
✔ renders the organ library with every organ
✔ server-renders the selected organ rather than an empty shell
✔ ships the viewer mount and a text equivalent for its hotspots
✔ carries no trace of the vinext starter it was scaffolded from
ℹ pass 6   ℹ fail 0

Full npm test (build + test) passes.

Also verified against #9. Assertions were deliberately chosen to hold on both main and the viewer branch — I merged #9 into a scratch branch and re-ran the suite, 6/6. So these two can merge in either order without conflict.

Should go in before #12, since a dependency bump needs a working test before it can be trusted.

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