test: cover the SSR and "use client" guarantees

[sidgaikwad]

Fixes #41.

The README makes two promises that nothing verified:

works out of the box in React Server Components environments (e.g. Next.js App Router) — it ships with the 'use client' directive and touches the DOM only inside effects

The whole suite runs in jsdom, so every test renders client-side, and the 'use client' banner is a tsup config detail — a bundler or config change could drop it and every existing test would still pass.

New test/ssr.test.tsx, running with @vitest-environment node

1. Asserts there is genuinely no DOM in that environment first — otherwise, if jsdom leaked in, the rest of the file would prove nothing.
2. renderToString the component, bare and again with every prop set. Effects don't run on the server, so a prop that reached the DOM during render rather than in an effect would throw here.
3. Asserts both built bundles start with 'use client';.

Verification

The banner assertion is not decorative — I deleted the banner line from tsup.config.ts, rebuilt, and it failed:

× ships the "use client" directive in both builds

It also skips cleanly rather than failing when dist/ is absent:

Tests  3 passed | 1 skipped (4)

That matters because the checks CI job runs npm run build before the tests (so it executes there), while the Node/React matrix jobs run npm test without building (so it skips).

• 4 new tests; 50 total; coverage still 100% across the board
• lint, typecheck clean

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