breaking: make object definitions exact by default and add isObjectWith

[gjbkz]

Summary

• make direct object definitions exact plain-object checks by default
• add the public isObjectWith() combinator for legacy open-shape validation
• document migration guidance and cover runtime, diagnostics, identity, and inference

Tests

• npx biome format --write (modified source files)
• npm run lint
• npm run test:type
• focused Node tests
• npm run test:unit
• npx c8 npm run test
• npm run build
• ESM and CommonJS package-entry smoke tests

Closes #435

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.63%. Comparing base (d4313bc) to head (aa07bda).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #442      +/-   ##
==========================================
+ Coverage   97.57%   97.63%   +0.06%     
==========================================
  Files          48       48              
  Lines        2472     2541      +69     
  Branches      248      260      +12     
==========================================
+ Hits         2412     2481      +69     
  Misses         60       60              

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