Parse URL-encoded form data.

[samuel-williams-shopify]

Summary

• Add a configurable Protocol::URL::FormData::Parser.
• Parse streaming bodies independently of HTTP metadata.
• Build nested form data through Protocol::URL::FormData::Nested, preserving absent and empty values.
• Expose incremental ordered pair enumeration with size, pair-count, and depth limits.

Testing

• sus test
• rubocop
• bake decode:index:coverage lib