fix: repair failing utility-library tests (16 → 0 failures)

[stooit]

Summary

Fixes all 16 failing tests across the TypeScript utility library. Final result: 60 pass / 0 fail, tsc --noEmit clean. No test files modified, no dependencies added.

Changes by file

• src/calculator.ts — divide now throws on division by zero.
• src/string-utils.ts — wordCount collapses consecutive whitespace ("hello world" → 2); implemented truncate (word-boundary cut, "..." counts toward maxLength, returns unchanged when within limit). Added a RangeError guard for non-finite/negative maxLength.
• src/task-manager.ts — implemented remove and update (return false for unknown id), and sortBy (priority high→medium→low; createdAt oldest-first).
• src/date-utils.ts — fixed formatRelative off-by-one at the 36-hour boundary.
• src/validator.ts — isEmail now accepts long TLDs and rejects leading/trailing/consecutive dots in the local part; isUrl handles URLs with a port.

Assumptions & notes

• Expected behaviour was derived directly from the existing test cases (the spec). Test files were left untouched.
• isEmail was tightened (dot as an explicit separator) beyond the minimal fix; quoted local parts and IP-literal domains remain unsupported by design.
• isUrl accepts loopback/localhost (required by tests). SSRF caveats for callers documented in the source.
• TaskManager.update/remove take no actor parameter — authorization is intentionally left to the caller/API layer.

Verification

```
bun test → 60 pass, 0 fail, 70 expect() calls
```