feat: add a union type-guard combinator

[gjbkz]

Summary

• add the generic union() TypeGuard combinator with zero-, one-, and many-guard semantics
• preserve branch diagnostics and stable issue ordering for validate() and validateAll()
• export and document the new API with runtime and type-inference coverage

Tests

• npm run lint
• npm run test:type
• node --test src/union.test.ts
• npm run test:unit
• npx c8 npm run test
• bun test src --coverage --coverage-reporter=lcov
• npm run build
• ESM and CommonJS package-entry smoke tests

Closes #437

[codecov]

Codecov Report

❌ Patch coverage is 98.71795% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.73%. Comparing base (cf19875) to head (90ddd5e).

Files with missing lines	Patch %	Lines
src/union.ts	98.68%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #444      +/-   ##
==========================================
+ Coverage   97.70%   97.73%   +0.02%     
==========================================
  Files          49       50       +1     
  Lines        2620     2698      +78     
  Branches      275      292      +17     
==========================================
+ Hits         2560     2637      +77     
- Misses         60       61       +1     

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