fix: emit error for spread arguments and rest parameters instead of generating invalid ir

[cs01]

Summary

• sum(...args) previously generated invalid LLVM IR (passed array pointer where double expected), causing cryptic opt errors
• function f(a, ...rest) similarly generated mismatched parameter types
• Both now emit clear compile-time errors: "Spread arguments (...) in function calls are not yet supported" and "Rest parameters (...rest) are not yet supported"
• Array spread literals ([...a, 4]) are unaffected and continue to work

Test plan

• New compile-error test fixtures for spread args and rest params
• npm run verify:quick passes (530 tests + Stage 1 self-hosting)
• Array spread in literals still works

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