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.

breadcast-caststream-sys/src/message_port_bridge.cc

@@ -0,0 +1,35 @@
+#include "message_port_bridge.h"
+
+namespace breadcast_caststream {
+
+MessagePortBridge::MessagePortBridge(void* user_data,
+                                     PostMessageCallback post_message)
+    : user_data_(user_data), post_message_(post_message) {}
+
+MessagePortBridge::~MessagePortBridge() = default;
+
+void MessagePortBridge::DeliverMessage(const std::string& source_id,
+                                       const std::string& message_namespace,
+                                       const std::string& message) {
+  if (client_) {
+    client_->OnMessage(source_id, message_namespace, message);
+  }
+}
+
+void MessagePortBridge::SetClient(Client& client) {
+  client_ = &client;
+}
+
+void MessagePortBridge::ResetClient() {
+  client_ = nullptr;
+}
+
+void MessagePortBridge::PostMessage(const std::string& destination_id,
+                                    const std::string& message_namespace,
+                                    const std::string& message) {
+  post_message_(user_data_, destination_id.data(), destination_id.size(),
+               message_namespace.data(), message_namespace.size(),
+               message.data(), message.size());
+}
+
+}  // namespace breadcast_caststream