Implement Cast Streaming mirroring, DLNA casting, daemon+GUI, and breadd integration

Builds out the full v1 scope: a vendored+patched openscreen subset for
low-latency Cast Streaming (Mirroring receiver 0F5096E8) alongside the
existing Cast V2/HLS and new DLNA/AVTransport casting paths, breadcastd's
Idle/Casting state machine with a private IPC socket, the breadcast GTK4
popup as a thin IPC client, and bread.cast.*/bread.command.cast.* breadd
integration (device discovery, start/stop, mirroring lifecycle events).
Also adds bakery/systemd/Forgejo CI packaging.

Validated end-to-end against a real Chromecast/Google TV: negotiated
Cast Streaming session, live pipeline playback, and daemon+GUI click-to-cast/
stop through the actual popup.

contrib/breadcastd.service

@@ -0,0 +1,19 @@
+[Unit]
+Description=breadcast screen-mirroring daemon
+Documentation=https://git.breadway.dev/breadway/breadcast
+# Start after the graphical session is ready so WAYLAND_DISPLAY is set
+After=graphical-session.target
+PartOf=graphical-session.target
+
+[Service]
+Type=simple
+ExecStart=%h/.cargo/bin/breadcastd
+Restart=on-failure
+RestartSec=2
+
+# Forward stdout/stderr to the journal so `journalctl --user -u breadcastd` works
+StandardOutput=journal
+StandardError=journal
+
+[Install]
+WantedBy=graphical-session.target