Exercise & verify the GLFW + wgpu path on Windows

[codymullins]

Problem

README Status says the Windows path "exists through GLFW and wgpu but is not yet exercised." Skyline is only tested on macOS. Starling cannot ship on Windows until the GLFW backend + wgpu surface (from HWND) are verified end to end.

Current state

• src/Skyline/GlfwWindowBackend.cs is portable but untested on Windows.
• Surface creation from the HWND via Silk.NET's native-window source → wgpu is untested.
• tests/Skyline.WindowedTests runs only on macOS; no Windows CI exists (see the CI issue).

What's needed / scope

• Build Skyline.slnx on Windows; run Skyline.Tests, Skyline.Gpu.Tests, and the Skyline.WindowedTests console harness.
• Verify: surface create→acquire→present→resize→readback; pointer/keyboard/text input; clipboard text round-trip; chrome modes map to GLFW windows.
• Confirm a sample (HelloWindow, TexturedQuad) runs on a real GPU adapter (DX12/Vulkan via wgpu).

Acceptance criteria

• WindowedTests exits 0 on Windows; one sample runs without GPU errors; input + clipboard verified.

Notes

Cursor, IME preedit, and a11y are tracked separately and will need Windows implementations later.