Add structured failure classification

[V3RON]

Summary

• add a structured failureClass to runner results with sensible defaults for existing failure types and origins
• let suites attach custom classes via assert.classify(...) and testCase.classifyFailure(...) so related assertion failures can be grouped across runs
• update built-in reporters to surface failure classes, including grouped standard output summaries and reporter metadata in JSON and GitHub Actions output

Testing

• pnpm vitest run test/runner/execute-runner.test.ts test/runner/execute-suite.reporter.test.ts test/reporters/json-summary.test.ts test/reporters/standard.test.ts
• pnpm typecheck
• pnpm build

Closes #3