fix: repair failing utility tests and implement missing functions

[stooit]

Summary

Fixes all 16 failing tests across the TypeScript utility library. Test suite now: 60 pass, 0 fail; tsc --noEmit clean.

Only source files were changed — no test files modified, no dependencies added.

Fixes by file

• src/calculator.ts — divide now throws on a zero divisor instead of returning Infinity.
• src/string-utils.ts — wordCount collapses consecutive whitespace (splits on /\s+/ after trim); truncate implemented per its TODO: unchanged when str.length <= maxLength, otherwise truncates at a word boundary with "..." counting toward maxLength.
• src/task-manager.ts — implemented remove (false on unknown id), update (partial field updates, false on unknown id), and sortBy (priority high→low, createdAt oldest-first, plus status).
• src/date-utils.ts — fixed off-by-one in formatRelative: day bucketing uses Math.round on the absolute hour difference (36h → "2 days ago").
• src/validator.ts — isEmail accepts long TLDs and subdomains; isUrl accepts URLs with an explicit port.

Assumptions

• truncate reserves 3 chars for the ellipsis and backs up to the last word boundary; a maxLength <= 3 guard avoids a negative slice budget.
• sortBy on status (in the type union but not under test) was implemented rather than left to throw.
• update uses per-field !== undefined checks so a partial changes object can't blank unmentioned fields.

Non-blocking notes for follow-up

• isEmail is a syntactic check only — pair with a confirmation flow before treating an address as verified.
• update accepts caller-supplied values without length/content bounds; worth a security pass if this feeds a service storing personal data.

Testing

• bun test → 60 pass, 0 fail
• bunx tsc --noEmit → exits 0