fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 failing tests in the utility library (now 60/60 passing) by implementing missing functionality and correcting edge-case bugs. No test files modified, no dependencies added.

Changes

• calculator.ts — divide now throws on division by zero (including -0) instead of returning Infinity.
• string-utils.ts — implemented truncate (truncates at a word boundary with "..." counted toward maxLength, returns unchanged when within limit); fixed wordCount to split on any whitespace run so consecutive spaces/tabs/newlines no longer inflate the count.
• task-manager.ts — implemented remove (true/false via Map.delete), partial update (only applies keys present in changes, guarded by !== undefined), and sortBy for priority/status/createdAt (sorts a copy, leaving Map order intact).
• date-utils.ts — fixed off-by-one in formatRelative: Math.floor → Math.round with Math.abs applied before rounding, giving symmetric past/future day counts. Removed the now-stale BUG: comment.
• validator.ts — isEmail TLD quantifier {2,4} → {2,} to accept long TLDs (e.g. .museum); isUrl now accepts explicit ports while keeping the http/https protocol allowlist (still rejects ftp://).

Verification

• bun test: 60 pass / 0 fail
• tsc --noEmit: clean
• Independent review performed; no blocking issues.

Assumptions / notes

• isEmail remains a deliberately loose regex — it satisfies the tests but is not a full RFC 5322 validator. For security-relevant use, verification-by-delivery is the durable control. Left as-is since no test required tightening.
• update treats { description: undefined } as "no change" (conventional Partial semantics); there's no path to clear a field, matching the tested contract.