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/cast/streaming/ssrc.cc

@@ -0,0 +1,42 @@
+// 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.
+
+#include "cast/streaming/ssrc.h"
+
+#include <random>
+
+#include "platform/api/time.h"
+
+namespace openscreen::cast {
+
+namespace {
+
+// These ranges are arbitrary, but have been used for several years (in prior
+// implementations of Cast Streaming).
+constexpr int kHigherPriorityMin = 1;
+constexpr int kHigherPriorityMax = 50000;
+constexpr int kNormalPriorityMin = 50001;
+constexpr int kNormalPriorityMax = 100000;
+
+}  // namespace
+
+Ssrc GenerateSsrc(bool higher_priority) {
+  // Use a statically-allocated generator, instantiated upon first use, and
+  // seeded with the current time tick count. This generator was chosen because
+  // it is light-weight and does not need to produce unguessable (nor
+  // crypto-secure) values.
+  static std::minstd_rand generator(static_cast<std::minstd_rand::result_type>(
+      Clock::now().time_since_epoch().count()));
+
+  std::uniform_int_distribution<int> distribution(
+      higher_priority ? kHigherPriorityMin : kNormalPriorityMin,
+      higher_priority ? kHigherPriorityMax : kNormalPriorityMax);
+  return static_cast<Ssrc>(distribution(generator));
+}
+
+int ComparePriority(Ssrc ssrc_a, Ssrc ssrc_b) {
+  return static_cast<int>(ssrc_a) - static_cast<int>(ssrc_b);
+}
+
+}  // namespace openscreen::cast