Document structural root path semantics.

[samuel-williams-shopify]

Summary

• document paths as tree traversals where components are nodes and separators are edges
• distinguish the structural root [""] from an explicitly separated root path ["", ""]
• explain why both serialize as / while retaining different component and local-path semantics
• preserve the useful root-boundary tests considered in Use a canonical root path representation. #70 without changing the representation

This follows the discussion in #70 and records the intended model to avoid conflating structural equality with serialized equality in future changes.

Testing

• bundle exec sus test/utopia/path.rb
• bundle exec rubocop lib/utopia/path.rb test/utopia/path.rb
• bundle exec bake decode:index:coverage lib