fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 previously-failing tests (60/60 now pass) by implementing missing functionality and correcting edge-case bugs across the utility library. No test files were modified and no dependencies were added.

Changes

• src/calculator.ts — divide now throws on division by zero instead of returning a poisoned Infinity/NaN.
• src/string-utils.ts — implemented truncate (truncates at a word boundary with "..." counting toward maxLength; returns unchanged when within limit; guards short/negative maxLength). Fixed wordCount to split on /\s+/ so consecutive spaces/tabs/newlines are handled.
• src/task-manager.ts — implemented remove and update (both return false for an unknown id; update applies only fields explicitly present) and sortBy (priority high > medium > low; createdAt oldest first).
• src/date-utils.ts — fixed off-by-one in formatRelative day rounding. Rounding now takes the absolute value before Math.round, so 36h resolves to "2 days" for both past and future dates (a review pass caught that rounding the signed value regressed future dates at .5-day boundaries).
• src/validator.ts — isEmail now supports arbitrary subdomain depth and long TLDs with anchored labels (still rejects a@-b.com, a@b..com). isUrl accepts URLs with a port (e.g. http://localhost:3000) while retaining the http/https scheme allow-list.

Verification

• bun test → 60 pass, 0 fail
• bunx tsc --noEmit → clean
• Changes reviewed by a review subagent; the one blocking regression it identified (future-date rounding) was fixed before commit.

Assumptions

• Email domain charset: the implementation validates ASCII alphanumeric + hyphen domain labels. Non-ASCII/IDN domains (e.g. user@münchen.de) that the old permissive regex accepted are now rejected. This matches the test contract; IDN support was never a requirement. Flagged here in case a consumer relies on it.
• sortBy ties on createdAt fall back to insertion order via stable sort, which equals creation order given createdAt is assigned in add.