fix: implement missing utilities and fix edge-case bugs to pass all tests

[stooit]

Summary

Makes all 60 tests pass (was 16 failing). Implemented missing functionality and fixed edge-case bugs across five utility modules. No test files modified, no new dependencies.

Changes

File	Fix
src/calculator.ts	divide now throws Error("Division by zero") when b === 0 (also catches -0) instead of returning Infinity/NaN.
src/string-utils.ts	Implemented truncate (word-boundary cut, "..." counted within maxLength, unchanged when it already fits) and fixed wordCount to collapse consecutive spaces.
src/task-manager.ts	Implemented remove/update (return boolean, false for unknown id) and sortBy (priority high>medium>low; createdAt oldest-first, stable).
src/date-utils.ts	Fixed formatRelative off-by-one (Math.round instead of Math.floor on the day boundary).
src/validator.ts	Fixed isEmail for long TLDs and isUrl for URLs with a port (now uses the URL constructor).

Verification

• bun test -> 60 pass / 0 fail
• tsc --noEmit clean

Assumptions

• Scope was strictly "fix what the tests require." A review pass surfaced several edge cases not covered by the current suite (e.g. formatRelative unit-boundary output, NaN on invalid dates, TaskManager accessors returning live references, truncate with negative maxLength). These are pre-existing/unreachable by tests and were intentionally left out to avoid behaviour changes beyond the test contracts; documented here for a possible hardening follow-up.

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