refactor: improve guest UUID generation and docs

[joshtrichards]

Summary

Some polishing of lib/guest.ts to improve the UUID helper implementation and docs:

• improve randomUUID() fallback behavior by using crypto.getRandomValues() (available even in Insecure Contexts like HTTP) before falling back as last resort to Math.random()
  • completely compatible; no BC introduced
• correct randomUUID() incorrect/misleading documentation (crypto.randomUUID is the preferred/best/most secure method)
• correct JSDoc for getGuestUser() (it does not return null)
• add brief JSDoc for class

This change preserves the API while making the UUID fallback logic more robust and the inline documentation more accurate.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[codecov]

Codecov Report

❌ Patch coverage is 12.50000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.36%. Comparing base (0b08876) to head (00b5eb3).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/guest.ts	12.50%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #950      +/-   ##
==========================================
- Coverage   70.00%   64.36%   -5.64%     
==========================================
  Files           4        4              
  Lines          80       87       +7     
  Branches       23       24       +1     
==========================================
  Hits           56       56              
- Misses         19       26       +7     
  Partials        5        5              

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