refactor: Remove NavKind, use Nav directly

[zharinov]

Summary

• Add Ord derive to Nav enum in plotnik-bytecode
• Remove redundant NavKind wrapper from verify.rs
• Use Nav directly in SemanticOp::Nav variant

Why

NavKind was a copy of Nav minus Epsilon, existing only because Nav lacked Ord. Adding Ord to Nav allows direct use, eliminating ~35 lines of boilerplate.