Build: add unit tests and PR build-test CI

[learncold]

Summary

• Add the first tracked src/ implementation skeleton for engine, domain, and application layers.
• Add CTest-based unit tests for FrameClock, EngineRuntime, and SafeCrowdDomain.
• Add a GitHub Actions workflow that configures, builds, and runs tests on pull requests.
• Update build/test documentation and PR verification guidance.

Why

• The repository had documented targets and architecture, but no tracked source files or test target.
• PRs were only validated for metadata, not for actual configure/build/test health.

Scope

• Add minimal engine/domain/application sources needed for a compilable baseline.
• Add safecrowd_tests and CTest integration.
• Add .github/workflows/ci.yml.
• Update presets and contribution guidance for test execution.