union type check

[cs01]

Summary

• Add a union type checker semantic pass that detects type alias unions with mixed LLVM representations (e.g., type Mixed = string | number) used as function/method parameters, which would silently miscompile and segfault at runtime
• Add a closure mutation checker semantic pass that detects reassignment of variables after they've been captured by a closure, which produces silently incorrect results since ChadScript closures capture by value

Both passes run before IR generation in LLVMGenerator.generateParts() and exit with a clear error message pointing to the offending line.

Test plan

• tests/fixtures/types/union-type-alias-error.ts — type alias union with mixed representations is a compile error
• tests/fixtures/types/union-non-nullable-error.ts — inline union with different representations is a compile error
• tests/fixtures/closures/closure-capture-mutation-error.ts — reassigning a captured variable is a compile error
• tests/fixtures/closures/closure-capture-by-value-ok.ts — reassignment before capture is allowed