Add for loop type inference support

[dak2]

Summary

• Add process_for_node in loops.rs to handle ForNode AST nodes, extracting element types from Array[T] / Range[T] collection literals and registering loop variables in the current scope
• Add ForNode dispatch in install.rs so for loops are no longer silently ignored
• Add Rust unit tests (5 cases: return type, variable typing, scope persistence, empty body, method calls) and Ruby integration tests (no-error and type-error detection)

Why

ForNode was completely unhandled in the AST installer — for x in [1, 2, 3] would silently skip type analysis, leaving loop variables untyped. This meant method calls like item.upcase inside for loops could not be validated, creating a gap in type coverage for Ruby code that uses for iteration.

Design decisions

• Returns nil (consistent with while/until) rather than the collection, since the return value of for is rarely used in practice
• No new scope — for in Ruby does not create a block scope, so the loop variable correctly persists after the loop
• Immediate edge propagation (genv.add_edge) for loop variable types, since they are determined at install time from literal collections

Test plan

• Rust unit tests: test_for_returns_nil, test_for_variable_type_from_array, test_for_variable_persists_after_loop, test_for_empty_body, test_for_with_method_call_in_body
• Ruby integration tests: test_for_loop_basic_no_error, test_for_loop_detects_type_error

🤖 Generated with Claude Code