deepEqual from scratch

Implement deepEqual(a, b) — structural equality:

1. Primitives compare by value; NaN equals NaN (unlike ===)
2. Arrays: same length, elements deep-equal in order
3. Plain objects: same key set (an explicit undefined value still counts as a key), values deep-equal
4. null equals only null — remember typeof null === 'object'
5. No JSON.stringify — it lies about key order, undefined, and NaN