fix: repair 16 failing tests across utility library

[stooit]

Summary

Fixes all 16 failing tests across five source files. Suite now: 60 pass / 0 fail, bunx tsc --noEmit exits 0. No test files modified, no dependencies added.

Changes

• src/calculator.ts — divide throws on a zero divisor (was returning Infinity). Covers 0 and -0.
• src/string-utils.ts — wordCount splits on /\s+/ after trim() (collapses consecutive whitespace); truncate implemented (was a stub): word-boundary truncation with "..." counted toward maxLength, returns input unchanged when within limit.
• src/task-manager.ts — remove delegates to Map.delete; update applies present fields in place with per-key !== undefined guards, returns false for unknown ids; sortBy uses an exhaustive switch with Record order tables, sorts a copy (Map order preserved), relies on stable sort for equal createdAt.
• src/date-utils.ts — formatRelative day count uses Math.round(Math.abs(diffHours) / 24), fixing the off-by-one (36h → "2 days ago").
• src/validator.ts — isEmail TLD quantifier relaxed {2,4} → {2,}; isUrl drops the port === "" clause (protocol check and try/catch retained).

Verification

• bun test → 60 pass / 0 fail
• bunx tsc --noEmit → exit 0
• Independent code review: approved, no blockers.

Notes / assumptions

• Stale // BUG: / // TODO: comments cleared where fixed.
• Non-blocking items left as-is by design (reviewer-noted): sortBy has no default (union-guarded by TS); update treats description: undefined as a no-op; isUrl now admits explicit ports (required by tests). None affect the test contract.