Add Lambda/Proc type inference support

[dak2]

Motivation

Lambda/Proc literals were silently ignored by the type checker, meaning type errors inside their bodies and on .call return values went undetected.

Changes

• Add Type::Proc variant with return_vertex and param_vertices for graph-based type propagation
• Add ScopeKind::Lambda so return inside lambdas targets the lambda scope, not the enclosing method
• Add GlobalEnv::enter_lambda() to create a lambda scope with its own merge vertex
• Add lambdas.rs handler: processes parameters, body, and wires the body result into the merge vertex
• Detect lambda {}, proc {}, and Proc.new {} in dispatch and route them through process_block_as_proc
• Handle .call on Type::Proc in MethodCallBox: connect return vertex and propagate arguments

Checked

• cargo test --lib passed
• bundle exec rake test passed

🤖 Generated with Claude Code