fix: repair failing utility tests and implement missing functions

[stooit]

Summary

Fixes all 16 failing tests across the utility library (now 60 pass / 0 fail) and implements previously-unimplemented functions. Only source files under src/ were changed — no test files modified, no dependencies added.

Changes

• calculator.ts — divide throws on division by zero instead of returning Infinity (also catches -0).
• string-utils.ts — wordCount collapses consecutive whitespace; truncate implemented to cut at a word boundary with an ellipsis counted toward maxLength.
• task-manager.ts — implemented remove, update (title/description/priority), and sortBy (priority high>medium>low, non-mutating).
• date-utils.ts — fixed off-by-one in formatRelative (round instead of floor; 36h → "2 days ago").
• validator.ts — isEmail accepts long TLDs (e.g. .museum); isUrl accepts URLs with a port (e.g. http://localhost:3000).

Verification

• bun test → 60 pass / 0 fail
• tsc --noEmit → clean
• Independent review: approved, no blockers

Assumptions / notes

• Scope limited to "fix only what the tests require". Review flagged two out-of-scope edge cases (fractional-minute rounding in formatRelative; negative maxLength in truncate) not covered by any test — left unchanged to avoid over-engineering.

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