Add causal person track GRU residual experiment

third_party/README.md

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+# Third-party reference code
+
+Reference repositories are inspected here but are not vendored into this
+project. They are not runtime dependencies.
+
+The track-GRU design was checked against the official EgoSpeak repository at
+revision `b5bee35e796d8f2d6aa7da2bf1b07836270f8bfb`:
+
+```bash
+git clone https://github.com/jun297/EgoSpeak.git third_party/EgoSpeak
+git -C third_party/EgoSpeak checkout b5bee35e796d8f2d6aa7da2bf1b07836270f8bfb
+```
+
+`third_party/EgoSpeak/` is intentionally ignored. No EgoSpeak source file is
+copied or imported by the project implementation.