Add vcpkg SDL3 to CI and update UI demo to use UiState; require SDL3 CONFIG in CMake

[Mixaill]

Motivation

• Ensure SDL3 is installed consistently across platforms and compilers in CI by integrating vcpkg and using vcpkg triplets.
• Make CMake prefer the config-mode package from vcpkg by using find_package(SDL3 CONFIG REQUIRED) to locate vcpkg-provided SDL3.
• Apply a small API refactor in the demo so the demo UI element uses UiState instead of UiApp and prints the rendering API name safely with ImGui::Text.

Description

• CI: updated .github/workflows/cmake-multi-platform.yml to add lukka/run-vcpkg@v11, install sdl3 via vcpkg, add vcpkg_triplet entries to the matrix, and pass -DCMAKE_TOOLCHAIN_FILE and -DVCPKG_TARGET_TRIPLET to CMake configure.
• CMake: changed find_package(SDL3 REQUIRED) to find_package(SDL3 CONFIG REQUIRED) in CMakeLists.txt to favor config-mode discovery from vcpkg.
• API change: modified src_demo/window_demo.h and src_demo/window_demo.cpp to change the signature from bool UiUpdate(UiApp&) to bool UiUpdate(UiState&), updated usage to state.app->GetRenderingApiName(), and adjusted the ImGui::Text call to use "%s" with c_str().

Testing

• No automated tests were executed as part of this change in the patch; the GitHub Actions workflow was updated so CI will run on the next push or pull request to main using the new vcpkg steps.

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Codex Task