Add global variable ($var) type tracking

[dak2]

Motivation

Reading an unregistered global variable ($unknown.upcase) silently dropped the expression from the type graph, producing no diagnostics. Ruby defines uninitialized globals as nil, so this was a false negative.

Changes

• Return nil-typed vertex for uninitialized global variable reads, matching Ruby semantics
• Register nil vertex in global variable map to avoid redundant allocations on repeated reads
• Remove dead class_vars field from Scope
• Fix inaccurate doc comment referencing non-existent finish_global_var_write
• Improve Rust test to verify actual type propagation (not just VertexId identity)
• Add integration tests: same-method write/read, cross-class access, multiple independent globals

Checked

• cargo test --lib — 33 tests passed

🤖 Generated with Claude Code