Clipboard: rich formats (image + HTML/MIME) beyond plain text

[codymullins]

Closes #21.

The clipboard was plain text only. A browser needs to copy/paste images and HTML. This adds typed clipboard read/write while keeping ClipboardText as the text convenience.

What changed

• AppWindow.GetClipboardData(mimeType) / SetClipboardData(mimeType, data) / ClipboardFormats → IWindowBackend.
• AppKit backend — maps text/plain, text/html, and image/png to their NSPasteboard UTIs and reads/writes typed NSData. ClipboardFormats enumerates which of those are present.
• GLFW backend — text only: text/plain round-trips through the GLFW clipboard string; other MIME types are dropped on write and return null on read; ClipboardFormats reports text/plain when text is present.

Tests

• Skyline.WindowedTests — typed text round-trip on a real GLFW window, null for an unsupported type, a dropped non-text write (no throw), and ClipboardFormats. Covers every GLFW clipboard branch.

Verification

• dotnet build Skyline.slnx clean (0 warnings); Skyline.Apple builds clean on the macos workload.
• Headless tests pass; windowed harness passes (92 checks); format-check.sh clean.
• Coverage: GlfwWindowBackend 100%; AppWindow 96.9% (matches main). Overall 98.1% unchanged.

Notes

• The AppKit typed read/write (NSPasteboard UTIs, image/html data) compiles on the macos workload but isn't exercised by the headless harness — the GLFW text path is the covered, verified one. Image/HTML round-trips want a hands-on macOS run.
• Branched on top of merged Input: carry modifier-key state on key and pointer events #29. Touches the window seam (IWindowBackend/AppWindow/GLFW/AppKit/FakeBackend), so it overlaps the open PRs in distinct regions — trivial rebases as they land.

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