fix: support HEAD requests for GET routes

[nvphungdev]

Summary

• register implicit HEAD handlers for GET routes so GET-only routes respond to HEAD
• preserve explicit HEAD routes by letting them replace the implicit fallback without double-registering uWS
• keep implicit HEAD fallback routes out of public route introspection/counts

Fixes #150

Verification

• npm test -- src/http/routing/route-registry.spec.ts
• npm run build
• npm run lint
• npm run format:check
• git diff --check

Scope note

• Narrowed this PR to HEAD requests to GET-only routes return 404 #150 only per maintainer feedback; Response body not suppressed for HEAD requests #151 response-body suppression is intentionally not included here.

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[VikramAditya33]

Can you please reduce down the changes to #150 only? As it allows another contributor to work on it too and you've opened up a straight PR without getting assigned so please comment under the issue so that I can assign you and your work doesn't conflict with any other person. Thank you

[nvphungdev]

Thanks for the guidance. I reduced this PR to cover #150 only and removed the #151 response-body suppression changes so it does not block another contributor.

I also commented on #150 for assignment/coordination.