fix: add zero-divisor guard to modulo to prevent srem UB

[cs01]

Summary

• Integer modulo (srem i64) with a zero divisor is undefined behavior in LLVM — can cause arbitrary wrong results or crashes
• Added zero-check guards to all three srem code paths (pure i64, known-integer doubles, dynamic integer detection)
• Zero divisor now returns 0 (i64 path) or NaN (double paths) instead of triggering UB

Test plan

• New modulo-zero.ts fixture covering basic modulo, negative operands, and float modulo
• npm run verify:quick passes (tests + self-hosting)

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