feat: implement Array.at() with negative index support

[cs01]

Summary

• Implement Array.at() ES2022 method for number[], string[], and object[] arrays
• Supports negative indices: arr.at(-1) returns last element, arr.at(-2) second-to-last, etc.
• Out-of-bounds returns NaN for number arrays, empty string for string arrays, null for object arrays
• Previously, calling .at() on an array gave a confusing "only available on strings" error

Test plan

• New test fixture arrays/array-at.ts covers positive, negative, and OOB indices for number and string arrays
• npm run verify:quick passes (tests + Stage 1 self-hosting)

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