Add WebRTC media transport to realtime TUI (#17058)

Adds the `[realtime].transport = "webrtc"` TUI media path using a new
`codex-realtime-webrtc` crate, while leaving app-server as the
signaling/event source.\n\nLocal checks: fmt, diff-check, dependency
tree only; test signal should come from CI.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/cli/build.rs

@@ -0,0 +1,5 @@
+fn main() {
+    if std::env::var("CARGO_CFG_TARGET_OS").as_deref() == Ok("macos") {
+        println!("cargo:rustc-link-arg=-ObjC");
+    }
+}