fix: repair edge-case bugs and implement missing utility functions

[stooit]

Summary

Fixes all 16 failing tests in the utility library (now 60 pass / 0 fail) by repairing edge-case bugs and implementing missing functions. Only the 5 source files were touched — no test files modified, no dependencies added.

Changes

• src/calculator.ts — divide now throws on division by zero instead of silently returning Infinity.
• src/string-utils.ts
  • wordCount trims and splits on \s+, so consecutive/leading/trailing whitespace collapses correctly.
  • truncate implemented — truncates at a word boundary, ellipsis (...) counts toward maxLength, returns unchanged when within the limit.
• src/task-manager.ts — implemented remove, update (partial field updates), and sortBy (priority high>medium>low, createdAt oldest-first, status).
• src/date-utils.ts — formatRelative day bucket uses Math.round instead of Math.floor, fixing the 36h → "2 days ago" off-by-one.
• src/validator.ts — isEmail accepts long TLDs (e.g. .museum); isUrl accepts URLs with a port (e.g. http://localhost:3000).

Verification

• bun test: 60 pass, 0 fail across 5 files.
• tsc --noEmit: clean.
• Independent review confirmed no regressions in behaviour outside test coverage.

Assumptions / notes

• sortBy("status") orders pending → in_progress → completed (matches the Status union declaration order); not covered by a test.
• Two latent edge cases in new code, out of scope for the tests and left as-is per "fix only what the tests require": truncate with a negative maxLength, and update clearing an optional description via undefined.

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