[Engine] implement generation-safe entity registry

[learncold]

Summary

• add a generation-safe Entity handle with index/generation identity and stream output
• implement EntityRegistry allocation, release, liveness, and signature storage APIs
• wire the new engine/test sources into CMake and add coverage for reuse, stale handles, and signature persistence

Related Issue

• Closes Task-Implement generation-safe Entity handle and EntityRegistry #6

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)

Risks / Follow-up

• None.