fix: emit error for x++/x-- at global scope instead of silently dropping

[cs01]

Summary

• x++ and x-- at global scope were silently dropped by both parsers, producing wrong values (variable unchanged)
• now both parsers detect unary expressions at global scope and emit a clear error with workaround suggestion
• increment/decrement works correctly inside functions — this only affects top-level code

Test plan

• verify:quick passes (tests + stage 1 self-hosting)
• confirmed compiler source doesn't use x++/x-- at global scope

🤖 Generated with Claude Code