fix: detect captured variable mutation inside closures

[cs01]

Summary

• arr.forEach((x) => { sum = sum + x; }) where sum is an outer variable was silently producing wrong output (sum stayed 0) because ChadScript captures by value
• The closure-mutation-checker only detected mutations AFTER capture (x = 2 after const f = () => x), not mutations INSIDE the closure body
• Fix: pass captured variable names into the arrow body walk so assignments to captured vars inside the closure are flagged as compile errors
• Users now get a clear error message instead of silently wrong behavior

Test plan

• New fixture closures/closure-inner-mutation.ts — forEach with captured var mutation triggers compile error
• All 621 tests pass
• Self-hosting verification passes (compiler itself doesn't use this pattern)

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