Add comprehensive test suite

Expand test coverage from 16 to 79 test cases organized into categories:

- Primitive values: strings, numbers, booleans, symbols, bigints
- Special values: NaN, +0/-0, Infinity, null, undefined
- Objects: empty, shallow equal, key order, inherited/non-enumerable props
- Arrays: empty, same elements, different lengths, sparse arrays
- Mixed types: array vs object, Date, RegExp, Map, Set, functions
- Customizer function: arguments, this binding, return value handling
- Edge cases: numeric keys, undefined values, many keys, circular refs,
  typed arrays, ArrayBuffer, Error, Promise, WeakMap/WeakSet

Tests document actual shallowequal behavior including that objects with
no enumerable own properties (Date, RegExp, Map, Set) compare as equal.

Author: claude