KDE-Wayland focus backend (KWin) #31

What to build

Implement KdeFocusBackend(PlatformBackend) for KDE Plasma on Wayland, analogous to the existing GnomeShellFocusBackend in daemon/deckd/platform.py. The shape of the backend (D-Bus names, polling cadence, returned AppInfo fields: app_id, wm_class, title, pid) should follow whatever spike #30 decides.

End-to-end behaviour: a user on KDE Plasma Wayland installs the deckd focus counterpart (KWin script or however the spike decides), runs the daemon with no GNOME extension, focuses a window, and the connected client flips to that app's layout automatically. just watch-focus reports sensible output on KDE.

Acceptance criteria

• KdeFocusBackend exists in daemon/deckd/platform.py and returns an AppInfo with the same fields the GNOME backend does (app_id / wm_class / title / pid where available).
• default_backend() dispatches to KdeFocusBackend on KDE Plasma Wayland sessions (detect via XDG_CURRENT_DESKTOP=KDE + XDG_SESSION_TYPE=wayland, per the spike).
• Backend handles the focus source missing (no script installed) gracefully — log a clear install hint analogous to the GNOME case, do not crash the daemon.
• Tests in tests/ exercise the KDE backend against a faked D-Bus / focus source, mirroring the GNOME backend's test shape.
• A just recipe (e.g. install-focus-kwin or similar) installs + enables the KWin script / counterpart, mirroring just install-focus-extension.
• The focus-watcher counterpart under packaging/ (e.g. packaging/kwin/) committed, licensed and structured to match repo conventions.
• README Focus watcher section updated to describe the KDE install/enable flow + just watch-focus usage on KDE.
• docs/SPIKES.md cross-references this delivery from the spike entry.

Blocked by

• Spike: KDE-Wayland active-window detection (focus watcher) #30 (Spike: KDE-Wayland active-window detection) — the implementation must use the path the spike decides.