fix: repair failing utility tests and implement missing functions

[stooit]

Summary

Fixes all 16 previously-failing tests (60/60 pass, tsc --noEmit clean). Changes are confined to src/; no test files were modified and no dependencies added.

Changes

• calculator.ts — divide now throws on a zero divisor (b === 0, catches -0) instead of returning Infinity.
• string-utils.ts — wordCount splits the trimmed string on /\s+/ (fixes consecutive spaces, tabs, newlines). truncate implemented: returns unchanged when str.length <= maxLength; otherwise reserves 3 chars for "...", cuts at the last word boundary within budget, trims trailing whitespace, and clamps negative maxLength to "".
• task-manager.ts — remove delegates to Map.delete; update returns false for unknown ids and applies only supplied keys; sortBy ranks priority (high>medium>low) and status, numeric compare on createdAt (stable sort preserves insertion order).
• date-utils.ts — day bucket uses Math.round(Math.abs(diffHours)/24) (abs before round), fixing the off-by-one so 36h reads "2 days ago" and future dates are symmetric.
• validator.ts — isEmail accepts arbitrarily many dot-separated domain labels with 2+ char TLDs (still rejects empty labels, trailing dots, bare domains); isUrl drops the spurious port === "" condition while keeping the protocol allowlist.

Testing

`bun test` → 60 pass / 0 fail. Independent review pass confirmed no logical/coincidental-pass issues (verdict: no blockers).

Assumptions & notes

• Fixed only what the tests require, per task constraints. Left `isUsername`/`isAustralianPhone` untouched (their tests already passed).
• `sortBy("status")` ordering (in_progress > pending > completed) is not covered by a test; chose "most active first".
• `isEmail`/`isUrl` are format validators, not sanitisers/SSRF gates — callers guarding a trust boundary need a separate host allowlist and output encoding.