fix: repair all failing tests in utility library

[stooit]

Summary

Fixes all 16 failing tests across the 5 utility modules. Result: 60 pass / 0 fail (bun test), tsc --noEmit clean. No test files modified, no dependencies added.

Changes

• src/calculator.ts — divide throws on a zero divisor instead of returning Infinity (b === 0 also catches -0).
• src/string-utils.ts — implemented truncate (returns unchanged within limit; reserves 3 chars for ellipsis; cuts at last word boundary within budget, hard-cuts when no space or maxLength <= 3, output length never exceeds maxLength). wordCount now splits on /\s+/ so consecutive whitespace collapses.
• src/task-manager.ts — implemented remove (bool via Map.delete), update (returns false for unknown id; applies only fields present in changes), and sortBy (priority high>medium>low, createdAt oldest-first; sorts a copy, does not mutate state).
• src/date-utils.ts — formatRelative day calc uses Math.round instead of Math.floor, fixing the off-by-one so 36h → "2 days ago".
• src/validator.ts — isEmail TLD quantifier relaxed {2,4} → {2,} (allows .museum); isUrl no longer requires an empty port, so http://localhost:3000 validates.

Verification

• bun test: 60 pass / 0 fail, stable across repeated runs
• tsc --noEmit: clean
• Reviewed by review subagent: verdict COMMENT, no blockers — confirmed fixes address root causes with general logic, not test-input pattern-matching, and no test files were touched.

Assumptions & notes (non-blocking)

• sortBy("createdAt") includes a numeric-id tiebreaker for determinism when two tasks land in the same millisecond. Relies on add() generating numeric string ids.
• sortBy status ordering (in_progress < pending < completed) is unspecified by tests — a reasonable default chosen for completeness.
• update uses !== undefined checks (patch semantics), so optional fields can't be cleared to undefined via a partial update.
• isEmail remains a permissive shape check, not RFC 5322 validation — consistent with the test expectations.