feat: add fromDiagnosis to derive narrowing guards from diagnoses

[gjbkz]

Summary

• add fromDiagnosis to derive narrowing predicates from diagnosis generators
• preserve private diagnosis metadata and one-pass structured validation
• document and test boolean short-circuiting, detailed issues, and type inference

Tests

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

Closes #439

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.76%. Comparing base (9dda6de) to head (e342353).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #446      +/-   ##
==========================================
+ Coverage   97.74%   97.76%   +0.02%     
==========================================
  Files          50       50              
  Lines        2703     2732      +29     
  Branches      293      296       +3     
==========================================
+ Hits         2642     2671      +29     
  Misses         61       61              

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