Verify SSR fixture hydration

[infeng]

Reproduction

After react-viewer@3.2.4 changed the CommonJS entry to export the Viewer component directly, the SSR fixture still read require("../dist/index").default. The fixture continued to compile, but browser hydration received an undefined component, removed the server-rendered root, and threw a runtime error.

Root cause and fix

• Consume the direct CommonJS component export in the SSR fixture.
• Add a jsdom hydration check that evaluates the built client bundle against server markup, clicks the fixture button, and asserts that Viewer opens.
• Include the hydration check in npm run verify after the SSR client build.

Verification

• npm ci --ignore-scripts
• npm run verify
• Chromium 149: initial SSR HTML retained the open button; hydration changed it to close; the Viewer image and labelled close control rendered with no page errors.
• Registry scan found only https://registry.npmjs.org/.