feat: handle Set-Cookie headers, add waitForFakeTime, and update deps

[KyleJune]

Summary

Fixes Set-Cookie header corruption when multiple cookies are set, adds waitForFakeTime testing utility for Deno's FakeTime compatibility with @testing-library/react, and updates dependencies including react-router 7.13.2.

Changes

• Fix Set-Cookie headers being merged with commas by Headers.entries() — use getSetCookie() with append instead across all header forwarding paths (renderDocument, redirect handler, error handler)
• Add waitForFakeTime utility to @udibo/juniper/utils/testing — a drop-in replacement for waitFor that works when FakeTime is active, using FakeTime.restoreFor to flush the fake timer queue
• Add unstable_url to test mock args for react-router 7.13.2 compatibility
• Use useRef in createRoutesStub to avoid recreating the router on every render
• Update dependencies (react-router, @std/testing, etc.)
• Update testing docs and skill with waitForFakeTime usage guidance

Testing

• Added tests for multiple Set-Cookie header handling in server.test.tsx
• Updated all template and example tests to use waitForFakeTime where FakeTime is active
• All template, example, and package tests pass

[github-actions]

🎉 This PR is included in version 0.4.0 🎉

The release is available on:

• JSR.io
• GitHub release

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