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/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "breadcast-caststream-sys"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+authors.workspace = true
+description = "Low-level FFI bindings to a pruned, vendored openscreen Cast Streaming sender (see vendor/openscreen/PATCHES.md)"
+links = "breadcast_caststream"
+build = "build.rs"
+
+[build-dependencies]
+cc = "1"
+pkg-config = "0.3"
+
+[dev-dependencies]
+tracing-subscriber = { workspace = true }