fix: implement missing utilities and fix edge-case bugs to pass all tests

[stooit]

Summary

Makes all 60 tests pass (was 44 pass / 16 fail) across the TypeScript utility library. Only src/ files changed — no test files modified, no dependencies added.

Changes

• calculator.ts — divide now throws on division by zero instead of returning Infinity.
• date-utils.ts — Fixed off-by-one in formatRelative: days computed as round(hours/24), so 36h correctly reads "2 days ago" while 24h stays "1 day ago".
• string-utils.ts — Implemented truncate (word-boundary cut, ellipsis counted within maxLength, result always ≤ maxLength); fixed wordCount to collapse consecutive whitespace (/\s+/).
• task-manager.ts — Implemented remove (boolean), update (partial, only applies defined keys, false for unknown id), and sortBy (priority high→medium→low, status, createdAt oldest-first).
• validator.ts — isEmail accepts subdomains and long TLDs (e.g. user@example.museum) while still rejecting empty labels/spaces; isUrl accepts URLs with ports (e.g. http://localhost:3000) with the scheme allowlist retained.

Verification

• bun test → 60 pass, 0 fail
• tsc --noEmit clean
• Independent review pass confirmed all five are genuine fixes (not test-shaped hacks) and each generalises beyond its assertions. Email regex checked for ReDoS — confirmed linear, not exponential.

Assumptions / notes

• Followed the task constraint "fix only what the tests require." A review flagged non-blocking follow-ups outside test scope: truncate with leading-whitespace-only input, isUrl validating syntax/scheme only (not SSRF destination safety), and a sortBy insertion-tiebreak that assumes numeric ids. None are regressions; left as-is to keep the change minimal.