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/build/buildflag.h

@@ -0,0 +1,17 @@
+// Minimal stand-in for Chromium's generated build/buildflag.h.
+//
+// The real header is produced per-target by GN's buildflag_header() rule,
+// which generates a `BUILDFLAG_<FLAG>()` macro per flag; BUILDFLAG(flag)
+// then token-pastes "BUILDFLAG_" with the flag name and calls it. Since this
+// vendored subset is compiled directly with the `cc` crate rather than
+// GN/Ninja, build_config.h defines those BUILDFLAG_<FLAG>() macros directly
+// for the Linux target breadcast always builds for. Not part of upstream
+// openscreen.
+#ifndef BUILD_BUILDFLAG_H_
+#define BUILD_BUILDFLAG_H_
+
+#define BUILDFLAG_CAT_INDIRECT(a, b) a##b
+#define BUILDFLAG_CAT(a, b) BUILDFLAG_CAT_INDIRECT(a, b)
+#define BUILDFLAG(flag) (BUILDFLAG_CAT(BUILDFLAG_, flag)())
+
+#endif  // BUILD_BUILDFLAG_H_