Add keyword argument support to type inference

[dak2]

Summary

Ruby's keyword arguments (def foo(name:, age: 0)) are widely used but were
previously ignored by the type checker, causing incomplete type propagation
when methods were called with keyword arguments.

Changes

• Parse keyword parameters (required and optional) in method definitions
  and register them as named vertices in the method registry
• Separate positional and keyword arguments at call sites by detecting
  KeywordHashNode in the argument list
• Propagate keyword argument types to parameter vertices by name matching
  in MethodCallBox, including through .new → initialize forwarding

Test plan

• Run cd rust && cargo test --lib to verify all Rust tests pass
• Run bundle exec rake test to verify all Ruby integration tests pass

🤖 Generated with Claude Code