IBus sends Unicode directly via commit_text(), bypassing keyboard layout
issues entirely. This provides the same reliable text injection on X11
that we already have on Wayland.
Changes:
- Add X11_IBUS desktop environment enum
- Prefer IBus on both X11 and Wayland (before falling back to xdotool)
- Add tests for X11 IBus preference and fallback behavior
This supersedes the layout-switching approach in PR #206, which only
worked on X11 and had overhead from layout switching.
Closes#199
