Version the immutable stylesheet - #2

Summary

• publish the changed stylesheet under the new immutable URL site-v1.css
• update the homepage, 404 page, build allowlist, and budget check together
• prevent returning visitors from combining cached pre-Canvas CSS with the reactive release

Validation

• ./scripts/check-repository.sh
• git diff --check
• clean deterministic build with only the versioned stylesheet in dist/

Release safety

The public production tree has not been changed. The existing release is hash-verified and backed up remotely before the staged production switch.