[Build] add unit tests and PR CI

[learncold]

Summary

• add the first tracked engine, domain, and application source skeleton under src/
• add a CTest-based safecrowd_tests target with unit coverage for FrameClock, EngineRuntime, and SafeCrowdDomain
• add a GitHub Actions workflow that configures, builds, and runs tests on PRs
• update presets and contribution guidance so test execution is part of normal verification

Related Issue

• Closes Build: add unit tests and PR build-test CI #40

Area

• Engine
• Domain
• Application
• Docs
• Build
• Analysis
• Chore

Architecture Check

• I kept the dependency direction application -> domain -> engine.
• I did not add Qt UI code to src/domain.
• I did not add domain or application dependencies to src/engine.
• I used src/ as the include root.

Verification

• cmake --preset windows-debug
• cmake --build --preset build-debug
• ctest --preset test-debug
• Not run (reason below)
• Local equivalent verification passed with cmake --preset local, cmake --build --preset local-build, and ctest --test-dir build/vs2022/local -C Debug --output-on-failure.
• The PR workflow will validate the repository-managed windows-debug / build-debug / test-debug path on GitHub.

Risks / Follow-up

• The tracked source tree is intentionally minimal and serves as a compilable baseline for future engine/domain work.
• If CI cache size becomes a problem, the vcpkg cache paths may need to be narrowed further.