test: add mutation testing (Stryker) for parser modules

[TechPMRyan]

Summary

The parser modules — extractControllers, extractServices, extractModules, extractModels — are the core of Tracer. Line coverage tells us the tests ran the code. Mutation testing tells us the tests would actually catch a regression. Without a mutation score, we cannot say with confidence that the test suite protects against changes that silently break parser output.

What needs to happen

• Install and configure Stryker for TypeScript (@stryker-mutator/core, @stryker-mutator/typescript-checker, @stryker-mutator/vitest-runner)
• Run mutation testing against src/parser/nestjs/ — all four extractor modules
• Establish a minimum acceptable mutation score (target: 75%+ as a starting baseline)
• Add a test:mutation script to package.json
• Document any surviving mutants that represent acceptable gaps

Why this matters

The parser is what engineers stake deployment decisions on. If a change breaks joinPaths, or the wrong decorator name check, or dependency extraction logic — and the test suite still passes — that is a trust problem. Mutation testing makes that gap visible before it ships.

Acceptance criteria

• Stryker configured and running against src/parser/nestjs/
• Mutation score reported per module
• Minimum score threshold enforced (fail if below target)
• test:mutation script added to package.json
• Any surviving mutants reviewed and either fixed or explicitly documented as acceptable