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/base.py

@@ -0,0 +1,41 @@
+"""What every receiver protocol has to provide."""
+from __future__ import annotations
+
+from ..devices import Device
+
+
+class BackendError(Exception):
+    pass
+
+
+class Backend:
+    kind = ""
+    #: Containers this protocol can be handed, best first.
+    containers: tuple[str, ...] = ("webm",)
+    #: Does it pull a plain HTTP stream (True) or an HLS playlist (False)?
+    progressive = True
+
+    def __init__(self, daemon) -> None:
+        self.daemon = daemon
+
+    async def play(self, device: Device, url: str, *, container: str, title: str) -> None:
+        raise NotImplementedError
+
+    async def stop(self, device: Device) -> None:
+        raise NotImplementedError
+
+    async def set_volume(self, device: Device, volume: float) -> None:
+        raise BackendError("this receiver has no volume control")
+
+    async def set_muted(self, device: Device, muted: bool) -> None:
+        raise BackendError("this receiver has no mute control")
+
+    async def transport(self, device: Device, action: str) -> None:
+        raise BackendError("this receiver has no transport control")
+
+    async def poll(self, device: Device) -> dict:
+        """Live status of the session: {state, app, volume, muted, error}."""
+        return {}
+
+    async def close(self) -> None:
+        return None