Add relative URL conversion.

[samuel-williams-shopify]

Summary

• Add Protocol::URL::Relative#relative_to to express root-relative URLs relative to a URL or path.
• Preserve query strings, fragments, and encoded path segments.
• Return already-relative references unchanged.
• Keep absolute URLs unchanged because relativizing them requires origin comparison.

Verification

• bundle exec sus: 377 passed, 554 assertions.
• bundle exec rubocop lib test: 20 files inspected, no offenses.