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.

vendor/rust_cast-0.21.0/build.rs

@@ -0,0 +1,22 @@
+use protobuf_codegen::{Codegen, Customize};
+use std::env;
+
+fn main() {
+    let generate_proto = env::var("GENERATE_PROTO").unwrap_or_else(|_| "false".to_string());
+    if generate_proto == "true" {
+        Codegen::new()
+            .out_dir("src/cast")
+            .inputs([
+                "protobuf/authority_keys.proto",
+                "protobuf/cast_channel.proto",
+            ])
+            .includes(["protobuf"])
+            .customize(Customize::default().gen_mod_rs(false))
+            .run()
+            .expect("protoc");
+    }
+
+    println!("rerun-if-env-changed=GENERATE_PROTO");
+    println!("rerun-if-changed=protobuf/authority_keys.proto");
+    println!("rerun-if-changed=protobuf/cast_channel.proto");
+}