feat: add structured validation results with first-error and all-errors functions

[gjbkz]

Summary

• add validate() and validateAll() with structured machine-readable issues
• export built-in issue codes through ValidationIssueCode
• collect nested object and array issues through private checker diagnostics
• preserve the existing boolean guard, ensure(), and TypeChecker.test() paths

Tests

• npm run lint
• npm run test:type
• npm run test:unit
• npm run test:example

Closes #434

[codecov]

Codecov Report

❌ Patch coverage is 98.17629% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.57%. Comparing base (373c09b) to head (53008e3).

Files with missing lines	Patch %	Lines
src/typeChecker.ts	97.44%	5 Missing ⚠️
src/validation.private.ts	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #441      +/-   ##
==========================================
+ Coverage   97.01%   97.57%   +0.55%     
==========================================
  Files          45       48       +3     
  Lines        2144     2472     +328     
  Branches      212      248      +36     
==========================================
+ Hits         2080     2412     +332     
+ Misses         64       60       -4     

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