Add dataset card and integrity manifest

DATASET_MANIFEST.json

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+{
+  "repository": "ThomasWalker1/paarbench-data",
+  "root": "pushobj_multishape",
+  "splits": {
+    "train": {
+      "episodes": 7200,
+      "file_count": 7206,
+      "total_bytes": 864106305,
+      "tree_sha256": "1f5e743c220cde93322b3735c3ba7caa161826bd86368bea6901c94616c4488d"
+    },
+    "val": {
+      "episodes": 800,
+      "file_count": 806,
+      "total_bytes": 97366292,
+      "tree_sha256": "ea2e04833041a334409778f36942e9c57bb97eb69100738bda71caed4f1e0a87"
+    }
+  }
+}

README.md

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+# Optional training dataset
+
+The PAARBench evaluation protocol does **not** require training trajectories. Its goal
+segments are tracked under `data/pushobj_eval/`, and evaluation reads fixed
+normalization metadata without opening the training dataset.
+
+The training release is provided for researchers who want to reproduce base-model or
+adapter training, or develop methods that explicitly use offline trajectories:
+
+```bash
+.venv/bin/python scripts/download_data.py
+```
+
+The downloader fetches the immutable benchmark release from the
+[`ThomasWalker1/paarbench-data`](https://huggingface.co/datasets/ThomasWalker1/paarbench-data)
+dataset repository into `data/pushobj_multishape/` and verifies it against
+`DATASET_MANIFEST.json`.
+
+## Contents
+
+The release contains only the nonredundant splits used by the training pipeline:
+
+| split | episodes | files | bytes |
+|---|---:|---:|---:|
+| `train` | 7,200 | 7,206 | 864,106,305 |
+| `val` | 800 | 806 | 97,366,292 |
+
+Each split contains `states.pth`, `velocities.pth`, `rel_actions.pth`,
+`abs_actions.pth`, `seq_lengths.pkl`, `shapes.pkl`, and one MP4 observation stream per
+episode under `obses/`. Redundant root-level copies and intermediate shards are not
+part of the release.
+
+The trajectories were generated in the PushObj simulator across the T, L, Z, and +
+shapes. No license is asserted here beyond the rights of the repository owner; users
+should review the dataset repository card and applicable upstream asset licenses
+before redistribution.