fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 previously-failing tests in the utility library. The suite is now 60 pass / 0 fail, and tsc --noEmit is clean. No test files were modified and no dependencies were added.

Changes

• calculator (src/calculator.ts) — divide now throws on division by zero.
• string-utils (src/string-utils.ts) — implemented truncate (cuts at a word boundary, with "..." counting toward maxLength, returns unchanged when str.length <= maxLength, and safely handles very short strings); fixed wordCount to collapse consecutive whitespace.
• task-manager (src/task-manager.ts) — implemented remove, update, and sortBy (priority high>medium>low; createdAt oldest-first).
• date-utils (src/date-utils.ts) — fixed off-by-one in formatRelative (rounding hours instead of flooring).
• validator (src/validator.ts) — isEmail accepts long TLDs; isUrl accepts URLs with a port.

Verification

• bun test → 60 pass / 0 fail (70 assertions)
• bunx tsc --noEmit → clean

Review notes

A review pass confirmed all fixes correct with no regressions. The flagged isEmail regex was tested for ReDoS and found to be linear (not vulnerable), and CRLF-injection payloads are rejected. Non-blocking hardening suggestions were raised (SSRF docstring caveat on isUrl, stricter local-part rules on isEmail) but are out of scope here — the task was to fix only what the tests require.

Assumptions

• Behaviour was driven strictly by existing test expectations; no behavioural changes beyond what tests specify.