Add Wayland global shortcuts through the XDG portal

[turinglambdaai]

Summary

• add a first-party asynchronous org.freedesktop.portal.GlobalShortcuts adapter for Linux
• report portal capability/version and explicit bound, cancelled, unavailable, or failed outcomes
• preserve compositor activation timestamps/tokens without adding an X11/XGrabKey fallback
• verify discovery, session creation, binding, and activation against a hermetic mock portal on a private D-Bus session
• document the compositor-command fallback for desktops without the portal

Validation

• git diff --check
• hermetic GlobalShortcuts integration test added to the Linux embedded round-trip gate
• full cross-platform CI required before merge

Closes #209