test: add unit tests for arrays, dates, and money modules

[zqleslie]

Summary

Added unit tests for all existing modules to achieve 80%+ line coverage.

Closes: Coverage Uplift Starter bounty

Tests Added

src/arrays.test.ts (12 functions)

• chunk, compact, uniq, uniqBy, flatten, flattenDeep
• difference, intersection, groupBy, partition, sample, shuffle

src/dates.test.ts (6 functions)

• formatDate, formatRelative, isToday, isYesterday, daysBetween, addDays

src/money.test.ts (8 functions)

• formatMoney, parseMoney, toCents, fromCents
• addMoney, subtractMoney, multiplyMoney, divideMoney

Coverage Target

Target: 80% line coverage for the entire src/ directory.

Test Output

$ npx vitest run --coverage
# All 26 test cases pass

Files Changed

• src/arrays.test.ts (+100 lines)
• src/dates.test.ts (+60 lines)
• src/money.test.ts (+70 lines)