fix: Number() returns NaN for non-numeric strings instead of 0

[cs01]

Summary

• Number("abc") and Number("") now correctly return NaN instead of 0
• Uses strtod endptr check (same pattern as parseInt/parseFloat fix in fix: parseInt/parseFloat return NaN for non-numeric strings #257)
• Adds test fixture covering valid numbers, negative, zero, invalid strings, and empty string

Test plan

• npm run verify:quick passes
• New test builtins/number-nan exercises all edge cases

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