feat: add macro integration tests and example file - #149

Summary

• Add examples/macros.vx demonstrating defmacro with unless and when macros
• Add end-to-end compilation tests in lib.rs verifying macros compile through the full pipeline (source → lex → parse → macro expand → type check → codegen → Go)
• Add interpreter-based integration tests (defmacro_unless_via_interpreter, defmacro_when_via_interpreter, defmacro_double_via_interpreter) proving macros work through the interpreter path

Test plan

• defmacro_unless_compiles — unless macro compiles to Go
• defmacro_when_compiles — when macro compiles to Go
• defmacro_stripped_from_go_output — defmacro forms absent from generated Go
• defmacro_with_hygiene_compiles — hygiene macro compiles correctly
• defmacro_multiple_macros_compile — multiple macros in one file
• defmacro_example_file_compiles — examples/macros.vx compiles end-to-end
• defmacro_unless_via_interpreter — unless macro via interpreter
• defmacro_when_via_interpreter — when macro via interpreter
• defmacro_double_via_interpreter — double macro via interpreter
• All 551 project tests pass
• cargo clippy -- -D warnings clean