fix: make all failing tests pass across utility library

[stooit]

Summary

Fixes all 16 failing tests (now 60 pass / 0 fail) by implementing missing functions and correcting edge-case bugs across five utility modules. No test files were modified and no dependencies were added.

Changes

• src/calculator.ts — divide now throws on a zero divisor instead of silently returning Infinity.
• src/string-utils.ts — implemented truncate (word-boundary clip with "..." counted toward maxLength, unchanged when str.length <= maxLength, clamped for small/negative maxLength); fixed wordCount to split on runs of whitespace after trimming.
• src/task-manager.ts — implemented remove (Map-backed true/false contract), update (applies only supplied fields, honouring falsy values like ""), and sortBy (priority/status rank maps, numeric createdAt; sorts a copy so Map order is preserved).
• src/date-utils.ts — day bucket uses Math.round on the absolute diff, fixing an off-by-one (e.g. 36h → "2 days ago") and making future dates round symmetrically.
• src/validator.ts — isEmail validates each domain label separately (fixes subdomains and long TLDs) with RFC 5321 length bounds; isUrl accepts a non-empty port while keeping the http/https-only restriction.

Verification

• bun test → 60 pass / 0 fail (was 44 pass / 16 fail).
• bunx tsc --noEmit → clean.
• Reviewed via subagent; two review findings folded in before commit (negative-maxLength clamp in truncate; corrected a misleading ReDoS rationale in the isEmail docstring).

Assumptions / notes

• truncate with maxLength <= 3 on a longer string hard-clips without an ellipsis (ellipsis alone would exceed the budget) — untested edge, chosen for the length guarantee.
• update uses !== undefined checks, so an optional field cannot currently be cleared by passing undefined — matches existing untested behaviour; left as-is.
• Status sort ordering (in_progress > pending > completed) is not test-covered; chosen as a sensible default.