Restore Track 2 variant index

Restore the Track 2 index README omitted by an overlapping add/delete operation in the first PR commit.

track_2/README.md

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+# Track 2: trajectory variants
+
+Track 2 contains the same 30 human-object interaction episodes in three parallel trajectory variants. All variants use the same episode order, 22,990-frame timeline, tasks, and four synchronized RGB views.
+
+| Variant | Directory | Human trajectory | Object trajectory and mesh |
+|---|---|---|---|
+| Baseline | [`baseline/`](baseline/) | Original four-view SOMA-X reconstruction | Original four-view pose and aligned mesh |
+| Synthetic random walk | [`synthetic_random_walk/`](synthetic_random_walk/) | Baseline with deterministic root SE(3) random walk | Baseline with an independent deterministic object SE(3) random walk; original mesh |
+| CARI4D reconstruction | [`cari4d_reconstruction/`](cari4d_reconstruction/) | Monocular CARI4D reconstruction converted to SOMA-X | Monocular CARI4D refined pose and its centered aligned mesh |
+
+Each directory is a complete LeRobot v2.1 dataset with its own `README.md`, `meta/`, `data/`, `videos/`, and `mesh/`. Select one variant directory as the dataset root. Do not concatenate variants: episode and global frame indices intentionally overlap so results can be compared frame by frame.
+
+The four video streams are byte-identical across variants. The synthetic variant is deterministic under the published seed and bounds. The CARI4D variant is reconstructed from the front-left video and masks; the other three streams are retained as synchronized observations.