feat: add narrow for composable type-guard refinements with diagnostics

[gjbkz]

Summary

• add the narrow() combinator for reusable subtype predicates
• preserve custom and fallback structured diagnostics without affecting boolean validation
• cover inference, interoperability, short-circuiting, nesting, and input identity

Tests

• npm run lint
• npm run test
• bun test src --coverage --coverage-reporter=lcov
• npm run build
• ESM and CommonJS package-entry smoke tests

Closes #436

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.70%. Comparing base (69b8662) to head (08f78a4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #443      +/-   ##
==========================================
+ Coverage   97.63%   97.70%   +0.07%     
==========================================
  Files          48       49       +1     
  Lines        2541     2620      +79     
  Branches      260      275      +15     
==========================================
+ Hits         2481     2560      +79     
  Misses         60       60              

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