fix: resolve 16 failing tests across utility modules

[stooit]

Summary

Fixes all 16 failing tests in the utility library. Test suite now passes 60/60 (was 44 pass / 16 fail). Only source files were changed — no test files modified, no dependencies added.

Changes by file

• src/calculator.ts — divide now throws RangeError on division by zero instead of returning Infinity (fail-fast on invalid input).
• src/string-utils.ts — implemented truncate (reserves 3 chars for "...", cuts to word boundary); fixed wordCount to split on /\s+/ so consecutive/other whitespace no longer inflates counts.
• src/task-manager.ts — implemented remove (Map.delete), update (partial update guarded by explicit key presence, returns false for unknown id), and sortBy (sorts a copy; data-driven priority/status rank maps; createdAt oldest-first).
• src/date-utils.ts — diffDays uses Math.round(Math.abs(diffHours) / 24) instead of Math.floor, fixing the off-by-one (36h → "2 days ago") symmetrically for past and future dates.
• src/validator.ts — isEmail TLD quantifier {2,4} → {2,} (accepts .museum etc.); isUrl drops the spurious empty-port check so http://localhost:3000 validates, while retaining the http/https allow-list.

Verification

• bun test → 60 pass / 0 fail
• tsc --noEmit clean (strict mode)
• Independent review pass confirmed tests were unmodified (60 tests present before and after) and that each fix is a genuine general implementation, not test-gaming.

Assumptions & notes

• Fixed only what the tests require, per task constraints. A review pass flagged optional, non-blocking polish for truncate — left unchanged as no test requires it.
• divide throwing is a behavioural change for any caller relying on native Infinity semantics; the test and the original // BUG: comment both require the throw.
• isEmail/isUrl remain format-shape checks, not RFC-grade or SSRF-safe validation — appropriate for their current use.