Tech upgrades

[tdecroyere]

Mechanical project cleanup before the backend-specific upgrade work.

Third-party dependencies

• update meshoptimizer, cgltf, Clay, fast_obj, stb, utest, and bc7enc_rdo
• remove the stale cimgui submodule entry
• keep DirectX-Headers, Vulkan-Headers/volk, and metal-cpp unchanged for their dedicated backend PRs

CMake

• use CMake-native file(ARCHIVE_EXTRACT) for downloaded packages
• clean the root configuration, presets, and Elemental target setup
• remove dead configuration blocks and the obsolete file.txt post-build command
• replace deprecated Wayland scanner code generation with private-code

CI

• update checkout, artifact, MSVC setup, and Ninja actions to current stable versions
• disable matrix fail-fast so every platform reports its result
• fix the aggregate CI gate so build failures, skipped tests, or cancellations cannot report green
• remove obsolete workflows under .github/old
• keep GraphicsTests explicitly manual because hosted runners do not provide the required GPU

Validation

Latest complete CI before the final warning-only Clay cleanup was green on all current hosted targets:

• Windows x64: build + tests ✅
• Linux x64: build + tests ✅
• macOS arm64: build + tests ✅

A fresh CI run is triggered by the final Clay compatibility cleanup.

Out of scope

• DirectX 12 / Agility SDK upgrades
• Vulkan / volk upgrades
• Metal / metal-cpp upgrades
• Apple packaging / iOS Simulator work
• architectural refactors