fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 failing tests (now 60 pass / 0 fail) by implementing missing functionality and correcting edge-case bugs across the utility library. No test files modified; no new dependencies added.

Changes

• src/calculator.ts — divide throws on division by zero (was returning Infinity).
• src/string-utils.ts — wordCount collapses consecutive whitespace via /\s+/; truncate implemented (word-boundary cut, "..." counting toward maxLength, unchanged when within limit, handles strings shorter than the ellipsis).
• src/task-manager.ts — implemented remove (returns bool), update (partial updates, ignores undefined keys), and sortBy (priority high>medium>low; createdAt oldest-first).
• src/date-utils.ts — fixed off-by-one in formatRelative (Math.round instead of Math.floor; 36h → "2 days ago").
• src/validator.ts — isEmail accepts multi-level subdomains and long TLDs; isUrl accepts URLs with ports (scheme still restricted to http/https).

Verification

• bun test → 60 pass, 0 fail
• npx tsc --noEmit → clean
• Code review passed (no blockers). ReDoS check on the isEmail regex confirmed linear time on adversarial input.

Assumptions & notes (deliberately out of scope — no test coverage)

Kept changes minimal per the "fix only what the tests require" constraint. Review flagged these as potential follow-ups, left unchanged to avoid untested scope creep:

• isEmail local-part uses an explicit allow-list; apostrophe/non-ASCII local parts (e.g. O'Brien, non-Anglicised names) are rejected. Worth widening for inclusivity if this gates a user-facing form.
• truncate can return bare "..." when the only space in the budget is leading whitespace.
• update cannot explicitly clear an optional field (uses !== undefined rather than "key" in changes).
• sortBy("status") ordering (in_progress>pending>completed) is implemented but untested/invented so the declared type isn't lying.
• isUrl validates scheme+syntax only, not destination safety (not an SSRF guard).