Exercise & verify the GLFW + wgpu path on Linux (X11/Wayland)

[codymullins]

Problem

The Linux path "exists through GLFW and wgpu but is not yet exercised." Starling cannot ship on Linux until the GLFW backend + wgpu Vulkan surface (X11 and/or Wayland) are verified.

Current state

• src/Skyline/GlfwWindowBackend.cs is portable; Silk.NET/GLFW choose X11 vs Wayland at runtime; never tested.
• No Linux CI; tools/cover.sh runs only on the current (macOS) host.

What's needed / scope

• Build Skyline.slnx on Linux; run the test vehicles incl. the WindowedTests harness on X11 (and Wayland if feasible).
• Verify: wgpu Vulkan surface from the X11 window / Wayland surface; surface create→acquire→present→resize→readback; pointer/keyboard/text; clipboard round-trip; resize/minimize/restore.

Acceptance criteria

• WindowedTests exits 0 on Linux (X11 at minimum); one sample runs without GPU errors/segfaults; input + clipboard verified.

Notes

X11 vs Wayland key-repeat/clipboard quirks may surface. Cursor/IME/a11y on Linux are tracked separately.