feat: unevalStream playground

[elliott-with-the-longest-name-on-github]

Stacked on #218. This is a copy of the #184 playground, adapted to the new unevalStream API so the two implementations can be compared side by side with completely separate playgrounds. Not meant to be merged as-is.

• The worker calls unevalStream synchronously, and the async-iterable tracking is gone from the worker and snapshot code, since feat: unevalStream #218 doesn't stream iterables (yet).
• The default example shows the new model: shared identity across head and tail blocks, delayed promises, a nested promise inside a settled value, a Vector class revived via the js-tag replacer, and a promise folded into head with Promise.resolve(…) from the replacer.
• Repo-level bits are the same as feat: playground #184: playground/* in the workspace, prettier-plugin-svelte and prettier ignores for .svelte-kit/.vercel.

pnpm install
pnpm --dir playground/uneval-stream dev

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