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/binds.json

@@ -0,0 +1,6 @@
+{
+  "_comment": "BOS-native keybind snippet (hyprland.lua + JSON config, not classic hyprland.conf — see CONTRIBUTING.md/README for context). There is no per-app self-registration mechanism today: merge this entry's contents into the 'bindings' array of BOS's own skel binds.json by hand, the same way breadclip's Super+V entry was added. Not auto-installed by bakery.",
+  "bindings": [
+    { "action": "exec", "command": "breadcast", "key": "C", "label": "Cast your screen (breadcast)", "category": "apps", "demo_cmd": "breadcast" }
+  ]
+}