Screencast: cast the desktop to Cast, DLNA and AirPlay receivers

An Omarchy shell plugin: a Python/asyncio daemon that discovers network
displays and serves an encoded screen capture for them to pull, plus a
Quickshell bar widget and panel to drive it.

The screen comes from the xdg-desktop-portal ScreenCast interface (asked
for every time, never remembered), goes through GStreamer, and is served
from a local HTTP port. Desktop audio is mixed in from the default
output's monitor. The container follows the receiver: WebM/VP8 for Cast,
MPEG-TS/H.264 for DLNA, HLS for AirPlay video.

The stream port has to be reachable from the LAN, and none of these
protocols can carry a credential, so the capability is the URL: a fresh
random path per session, refused to anything off the LAN, capped at four
concurrent readers.

daemon/screencast/backends/__init__.py

@@ -0,0 +1,10 @@
+from .base import Backend, BackendError
+from .cast import CastBackend
+from .dlna import DlnaBackend
+from .airplay import AirPlayBackend
+
+__all__ = ["Backend", "BackendError", "CastBackend", "DlnaBackend", "AirPlayBackend", "for_kind"]
+
+
+def for_kind(kind: str):
+    return {"cast": CastBackend, "dlna": DlnaBackend, "airplay": AirPlayBackend}.get(kind)