Allow URL component assignment.

[samuel-williams-shopify]

Summary

• Allow mutable relative URLs to replace query and fragment components.
• Allow mutable absolute URLs to replace scheme and authority components.
• Allow mutable references to replace structured parameters without changing the encoded query.
• Preserve the existing immutable behavior after freezing.

Testing

• bundle exec bake test
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib