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/vendor/openscreen/platform/api/task_runner.h

@@ -0,0 +1,64 @@
+// Copyright 2019 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef PLATFORM_API_TASK_RUNNER_H_
+#define PLATFORM_API_TASK_RUNNER_H_
+
+#include <future>
+#include <utility>
+
+#include "platform/api/time.h"
+
+namespace openscreen {
+
+// A thread-safe API surface that allows for posting tasks. The underlying
+// implementation may be single or multi-threaded, and all complication should
+// be handled by the implementation class. The implementation must guarantee:
+// (1) Tasks shall not overlap in time/CPU.
+// (2) Tasks shall run sequentially, e.g. posting task A then B implies
+//     that A shall run before B.
+// (3) If task A is posted before task B, then any mutation in A happens-before
+//     B runs (even if A and B run on different threads).
+class TaskRunner {
+ public:
+  using Task = std::packaged_task<void()>;
+
+  virtual ~TaskRunner() = default;
+
+  // Takes any callable target (function, lambda-expression, std::bind result,
+  // etc.) that should be run at the first convenient time.
+  template <typename Functor>
+  inline void PostTask(Functor f) {
+    PostPackagedTask(Task(std::move(f)));
+  }
+
+  // Takes any callable target (function, lambda-expression, std::bind result,
+  // etc.) that should be run no sooner than `delay` time from now. Note that
+  // the Task might run after an additional delay, especially under heavier
+  // system load. There is no deadline concept.
+  template <typename Functor>
+  inline void PostTaskWithDelay(Functor f, Clock::duration delay) {
+    PostPackagedTaskWithDelay(Task(std::move(f)), delay);
+  }
+
+  // Implementations should provide the behavior explained in the comments above
+  // for PostTask[WithDelay](). Client code may also call these directly when
+  // passing an existing Task object.
+  virtual void PostPackagedTask(Task task) = 0;
+  virtual void PostPackagedTaskWithDelay(Task task, Clock::duration delay) = 0;
+
+  // Return true if the calling thread is the thread that task runner is using
+  // to run tasks, false otherwise.
+  virtual bool IsRunningOnTaskRunner() = 0;
+
+  // Posts a task to delete `object`.
+  template <class T>
+  void DeleteSoon(const T* object) {
+    PostTask([object] { delete static_cast<const T*>(object); });
+  }
+};
+
+}  // namespace openscreen
+
+#endif  // PLATFORM_API_TASK_RUNNER_H_