fix: repair failing tests across utility modules

[stooit]

Summary

Fixes all 16 failing tests across the utility library. Only source files under src/ were changed — no test files modified, no dependencies added.

Result: 60 pass / 0 fail; tsc --noEmit clean.

Changes

• calculator — divide throws on division by zero (was returning Infinity). b === 0 also covers -0.
• string-utils — implemented truncate (returns unchanged when within limit; ellipsis counts toward maxLength; word-boundary aware; handles maxLength <= 3). wordCount trims and splits on /\s+/, so consecutive spaces/tabs/newlines don't inflate the count.
• task-manager — implemented remove (false for unknown id), update (applies only provided fields; false for unknown id), and sortBy (priority high>medium>low; createdAt oldest-first). sortBy sorts a copy and does not mutate internal state.
• date-utils — formatRelative off-by-one fixed via Math.round(Math.abs(diffHours)/24); 36h now reports "2 days ago".
• validator — isEmail accepts long TLDs (e.g. .museum) via a {2,63} quantifier plus a 254-char length guard; isUrl accepts URLs with a port (protocol allowlist retained, so ftp:/javascript:/data:/file: still rejected).

Assumptions / notes

• The date-utils fix also silently corrected a previously-untested future-date skew: e.g. 50h future was "in 3 days", now correctly "in 2 days". Improvement, called out so it isn't mistaken for a regression.
• update uses !== undefined checks, so it cannot clear an optional field — matches the common convention and no test requires clearing.
• Verified independently: no test files, package.json, or tsconfig.json changed.

Out of scope (flagged, not fixed)

• isUrl accepts any http/https host including loopback/private addresses — if ever used to guard a server-side fetch, that's an SSRF vector needing separate host allowlisting.
• Pre-existing isEmail looseness (user@example..com) is unchanged.