fix: implement missing utilities and correct edge-case bugs

[stooit]

Summary

Fixes all 16 failing tests in the utility library. bun test now reports 60 pass / 0 fail (was 44/16) and npx tsc --noEmit is clean. No test files, config, or dependencies were touched — only src/.

Changes

File	Change
calculator.ts	divide throws on b === 0 (was returning Infinity)
string-utils.ts	wordCount trims then splits on /\s+/; truncate implemented with word-boundary cut
task-manager.ts	remove (Map.delete), update (per-field !== undefined guards), sortBy (typed rank maps with an explicit id tie-break)
date-utils.ts	day count now Math.round(abs(diffHours)/24), fixing an off-by-one
validator.ts	email TLD bound {2,4}→{2,} (accepts .museum); dropped the url.port === "" conjunct (accepts http://localhost:3000)

Verification

• bun test → 60 pass, 0 fail
• npx tsc --noEmit → clean (exit 0)
• Diff scoped to 5 src/ files only

Assumptions & notes

• isEmail/isUrl were intentionally loosened to satisfy the tests. They are format checks, not sanitisers — isUrl now accepts loopback/internal hosts and credentials. Docblocks note this so the relaxation isn't mistaken for a security control; any server-side-fetch consumer needs its own allowlist (SSRF).
• update() guards each field with !== undefined, so passing undefined is a no-op rather than clearing a field. Flagged in review as a possible follow-up if explicit clearing is ever needed.
• Pre-existing (out of scope): formatRelative has a minor discontinuity in the sub-24h branch (23.5h renders as "24 hours ago"). Not introduced by this change.

Review

Reviewed by the review subagent: verdict COMMENT, no blockers. All warnings are consumer/edge concerns documented above.