fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 previously-failing tests across the utility library (was 44 pass / 16 fail → now 60 pass / 0 fail). No test files modified, no new dependencies.

Changes by file

• src/task-manager.ts — implemented three stub methods: remove (Map delete, returns false for unknown id), update (Object.assign merge, returns false for unknown id), and sortBy (priority via {high:0, medium:1, low:2} rank map, or ascending createdAt).
• src/calculator.ts — divide now throws on division by zero.
• src/date-utils.ts — relative-time formatting off-by-one: Math.floor → Math.round so 36h reports "2 days ago". Added a comment documenting the rounding intent to prevent regression.
• src/string-utils.ts — implemented truncate (word-boundary truncation with "..." counting toward maxLength; returns unchanged when within limit); fixed wordCount for consecutive spaces.
• src/validator.ts — isEmail accepts long TLDs (e.g. .museum); isUrl accepts an optional port (e.g. http://localhost:3000).

Verification

• bun test → 60 pass / 0 fail / 70 expect() calls.
• Reviewed for tests-passing hacks: the Math.round change matches the source's own documented intent (nearest-day semantics), and the validator regex changes were checked against the reject cases the tests expect.

Assumptions

• Fixed only what the tests require; behaviour matched to test expectations and existing TODO comments in the source.