AgentSpace.Avalonia.PrimarySelection

X11 PRIMARY-selection clipboard for Avalonia apps. Avalonia's IClipboard exposes the CLIPBOARD selection only; this adds the PRIMARY half (highlight-to-copy, middle-click-to-paste) via direct libX11 P/Invoke — no subprocess, no reflection. Ships a Linux X11 implementation under runtimes/linux/ with a stub assembly for Windows and macOS; Windows/macOS binaries contain zero libX11 metadata.