fix: JSON.stringify integers, implement number array join, add NaN/Infinity

[cs01]

Summary

• JSON.stringify integers: JSON.stringify({count: 42}) now outputs 42 instead of 42.0, matching JavaScript behavior. Fixed in yyjson bridge by detecting integer-valued doubles.
• Number array join: [1,2,3].join(", ") was returning empty string — the numeric join was a stub. Implemented proper loop with snprintf and integer detection.
• NaN and Infinity globals: NaN and Infinity are now recognized as global constants, enabling Number.isNaN(NaN), Number.isFinite(Infinity), etc.

Test plan

• builtins/json-stringify-integers.ts — JSON integer and float formatting
• arrays/number-array-join.ts — number array join with commas, floats, empty, single
• builtins/nan-infinity.ts — NaN/Infinity with Number.isNaN/isFinite/isInteger
• Updated json-stringify-class.ts and context-json-array.ts for correct integer format
• npm run verify:quick passes (tests + Stage 1 self-hosting)

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