fix(utils): resolve failing tests and implement missing functionality

[stooit]

Summary

Fixes all 16 failing tests across the utility library and implements the missing functions. All 60 tests now pass (was 44 pass / 16 fail); tsc --noEmit is clean. No test files modified, no dependencies added.

Changes

• src/calculator.ts — divide now throws on division by zero instead of silently returning Infinity.
• src/string-utils.ts
  • wordCount splits on whitespace runs (/\s+/), fixing consecutive spaces (and tabs/newlines).
  • truncate implemented: returns input unchanged when within maxLength; otherwise truncates at a word boundary with "..." counting toward maxLength. Whitespace-aware boundaries (tab/newline, for parity with wordCount), and the ellipsis contract is honoured down to maxLength === 3.
• src/task-manager.ts — implemented remove (true/false on presence), update (partial title/description/priority), and sortBy (priority high>medium>low, createdAt oldest-first, status by lifecycle). sortBy sorts a copy, preserving internal order.
• src/date-utils.ts — formatRelative uses Math.round (not Math.floor), so 36h rounds to "2 days ago". Symmetric for future dates.
• src/validator.ts — isEmail accepts long TLDs (e.g. .museum) and matches subdomains explicitly (net stricter — now rejects user@example..com); isUrl accepts URLs with a port (protocol allowlist retained as the actual control).

Verification

• bun test -> 60 pass, 0 fail
• tsc --noEmit -> clean
• Reviewed: no blocking issues; the two consistency warnings raised in review (whitespace boundaries in truncate, ellipsis at small maxLength) were addressed.

Assumptions

• sortBy("status") has no test coverage; implemented as lifecycle order pending -> in_progress -> completed.
• isEmail/isUrl remain pragmatic format checks (regex / protocol allowlist), suitable as input-format hints rather than authoritative gates.