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/Cargo.toml

@@ -48,6 +48,7 @@ members = [
     "ollama",
     "process-hardening",
     "protocol",
+    "realtime-webrtc",
     "rollout",
     "rmcp-client",
     "responses-api-proxy",
@@ -149,6 +150,7 @@ codex-otel = { path = "otel" }
 codex-plugin = { path = "plugin" }
 codex-process-hardening = { path = "process-hardening" }
 codex-protocol = { path = "protocol" }
+codex-realtime-webrtc = { path = "realtime-webrtc" }
 codex-responses-api-proxy = { path = "responses-api-proxy" }
 codex-response-debug-context = { path = "response-debug-context" }
 codex-rmcp-client = { path = "rmcp-client" }