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/time.h

@@ -0,0 +1,37 @@
+// Copyright 2018 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_TIME_H_
+#define PLATFORM_API_TIME_H_
+
+#include <chrono>
+
+#include "platform/base/trivial_clock_traits.h"
+
+namespace openscreen {
+
+// The "reasonably high-resolution" source of monotonic time from the embedder,
+// exhibiting the traits described in TrivialClockTraits. This class is not
+// instantiated. It only contains a static now() function.
+//
+// For example, the default platform implementation bases this on
+// std::chrono::steady_clock or std::chrono::high_resolution_clock, but an
+// embedder may choose to use a different source of time (e.g., the embedder's
+// time library, a simulated time source, or a mock).
+class Clock : public TrivialClockTraits {
+ public:
+  // Returns the current time.
+  static time_point now() noexcept;
+};
+
+// Returns the number of seconds since UNIX epoch (1 Jan 1970, midnight)
+// according to the wall clock, which is subject to adjustments (e.g., via NTP).
+// Note that this is NOT necessarily the same time source as Clock::now() above,
+// and is NOT guaranteed to be monotonically non-decreasing; it is "calendar
+// time."
+std::chrono::seconds GetWallTimeSinceUnixEpoch() noexcept;
+
+}  // namespace openscreen
+
+#endif  // PLATFORM_API_TIME_H_