Reorganize PRK.Client test projects by production boundary

Clean up the PRK.Client test layout so each test project represents the production assembly or integration boundary it verifies. Create direct client test coverage, keep graphics-engine tests scoped to the graphics engine, keep allocator unit tests scoped to the allocator, rename or relocate Vulkan renderer integration tests appropriately, and provide one discoverable client-focused solution entry point. Preserve test behavior and avoid duplicating shared test helpers.\n\nAcceptance criteria:\n- Client-owned tests run from a dedicated PRK.Client test project.\n- Graphics-engine tests no longer carry client-only dependencies or tests.\n- Memory-allocator test projects contain only allocator tests; renderer/Vulkan integration tests have an accurate home.\n- A solution under PRK.Client includes the client production and test projects.\n- All reorganized test projects build and their applicable tests pass.\n\nPull request: #2656 https://git.project-rk.com/prk/engines/pulls/2656