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