fix(rewrites): serve static files from public/ when rewrite target is a .html path

[yunus25jmi1]

Fixes #199.

Problem

When next.config rewrites() map a clean URL to a static .html file in public/ (a common pattern for serving pre-built static pages like /auth/no-access → /auth/no-access.html), vinext was returning 404 because no Next.js route matches .html paths.

Root Cause

After afterFiles rewrites resolve a path to *.html, the routing logic in all three server paths would find no matching app/pages route for the .html URL and return 404, without ever checking the filesystem's public/ directory.

Fix

Three server paths updated:

1. Pages Router dev (packages/vinext/src/index.ts): After afterFiles/fallback rewrites, before the final handler() call, check if the resolved URL points to a file in public/ and serve it directly.

2. App Router dev (packages/vinext/src/server/app-dev-server.ts): The generated RSC entry now checks public/ for the rewritten pathname before rendering the 404 page. Added a new optional root parameter to generateRscEntry() so the public directory path is embedded in the generated virtual module.

3. Production server (packages/vinext/src/server/prod-server.ts): After afterFiles rewrites and after fallback rewrites produce a path with a file extension, tryServeStatic() is called against the built clientDir (which contains public/ files) before passing to SSR.

Also added a staticMimeType() helper in index.ts to ensure correct Content-Type headers.

Tests

• Integration tests in app-router.test.ts and pages-router.test.ts: GET /static-html-page (rewritten → /static-html-page.html) returns 200 with correct HTML content and text/html Content-Type.
• Unit test in app-router.test.ts: generateRscEntry() embeds the public/ path in the generated code.
• Fixture public/static-html-page.html files added to both app-basic and pages-basic fixtures; next.config rewrites updated.

What async rewrites() flat array does

Per Next.js semantics, when rewrites() returns a flat array, all rules go into afterFiles. vinext already handles this correctly via resolveNextConfig(). This PR fixes the serving of the rewritten destination when it's a static file.