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/util/bit_vector.h

@@ -0,0 +1,66 @@
+// Copyright 2026 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef UTIL_BIT_VECTOR_H_
+#define UTIL_BIT_VECTOR_H_
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include <bit>
+#include <limits>
+#include <vector>
+
+#include "util/osp_logging.h"
+
+namespace openscreen {
+
+// A simple dynamic bit vector implementation using C++20 <bit> and std::vector.
+// This is used for tracking packet transmission flags in the Sender.
+class BitVector {
+ public:
+  enum Fill : bool { SET = true, CLEARED = false };
+
+  BitVector() noexcept = default;
+  BitVector(size_t size, Fill fill);
+
+  ~BitVector() = default;
+
+  BitVector(BitVector&& other) noexcept = default;
+  BitVector& operator=(BitVector&& other) noexcept = default;
+
+  BitVector(const BitVector& other) = default;
+  BitVector& operator=(const BitVector& other) = default;
+
+  [[nodiscard]] size_t size() const noexcept { return size_; }
+
+  void Resize(size_t size, Fill fill);
+
+  void Set(size_t pos) {
+    OSP_CHECK_LT(pos, size_);
+    v_[pos / kBitsPerWord] |= (uint64_t{1} << (pos % kBitsPerWord));
+  }
+
+  void Clear(size_t pos) {
+    OSP_CHECK_LT(pos, size_);
+    v_[pos / kBitsPerWord] &= ~(uint64_t{1} << (pos % kBitsPerWord));
+  }
+
+  [[nodiscard]] bool IsSet(size_t pos) const {
+    OSP_CHECK_LT(pos, size_);
+    return (v_[pos / kBitsPerWord] >> (pos % kBitsPerWord)) & 1;
+  }
+
+  [[nodiscard]] size_t FindFirstSet() const;
+
+ private:
+  static constexpr size_t kBitsPerWord = std::numeric_limits<uint64_t>::digits;
+
+  std::vector<uint64_t> v_;
+  size_t size_ = 0;
+};
+
+}  // namespace openscreen
+
+#endif  // UTIL_BIT_VECTOR_H_