[viktorprogger]

Summary

• Add public Assert::success() and Assert::failure() entry points for userland custom assertions.
• Reuse Testo's existing assertion history, failure reporting, expectation handling, and fiber isolation.
• Make AssertionException and ComparisonFailure supported public failure records, with validation before history mutation.
• Add feature, negative, and concurrency coverage; document the API and the intentionally unsupported Rector conversion.

Closes #275

Verification

• PHP 8.4.5 container lint for the changed PHP files: passed.
• Focused Testo suite:
  vendor/bin/testo --json --path='plugin/assert/tests/Feature/CustomAssertionsTest.php'
  — passed: 11 tests, 57 assertions.
• git diff --check: passed.
• The PHP 8.2 container is available for linting, but the installed Composer dependencies require PHP >= 8.4.1.
• The full assertion suite was not used as a local merge gate; a broader container run previously aborted in an existing Revolt isolation test. The focused custom-assertion suite is green.