docs: describe checker caching and type guard interop accurately

[gjbkz]

Requested by Kei · project thread

Before: the README advertised "Zero runtime overhead — Results are cached via WeakMap", although only checker construction is cached and every call still validates the input, and its intro implied Zod users must keep schemas and interfaces in sync by hand.

After: the intro drops the Zod comparison and says checkers are ordinary type guards that compose with existing predicates. The features list gains an "Interoperates with existing type guards" item pointing at typeChecker, narrow, union and fromDiagnosis (the Usage section already has examples of each), and "Cached checker construction" says the cache is keyed by the definition object and that validation results are not cached.

Closes #451

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https://claude.ai/code/session_019BJENnU8SXcDH2Tcx5EWE4

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.46%. Comparing base (25c62a4) to head (3c23185).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

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  Lines        3057     3057           
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  Hits         3010     3010           
  Misses         46       46           
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[gjbkz]

Reviewed commit 195d6de2cda7bb57fa334634a03ec4d7e92d9fbe.

No actionable issues found in this documentation change.

The updated README correctly distinguishes cached checker construction from per-call input validation and describes interoperability with standard TypeGuards without implying that Zod requires separately maintained interfaces. I checked the wording against the checker factory/cache implementation and the existing usage examples.

Validation: the GitHub Actions test workflow for this commit passed. This PR only changes prose; no new local runtime tests were run for this review.