Add dataset card and train split mapping

README.md

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+---
+configs:
+  - config_name: default
+    data_files:
+      - split: train
+        path: data/chunk-*/file-*.parquet
+---
+
+# CTR250fixed
+
+Merged LeRobot v3 dataset for the fixed-start CTR ring-placement task.
+
+- Task: Pick up a yellow ring and place it onto a red post.
+- Episodes: 355
+- Frames: 163,768
+- FPS: 30
+- Cameras: Cam2 and Cam4 at 960 x 960; Cam5 wrist camera at 400 x 400
+- Pilot dataset `CTR250fixed-test` is excluded.
+
+The 12 timestamped source datasets are retained separately for provenance. Merge provenance and validation are stored in `meta/ctr_merge_manifest.json` and `meta/ctr_merge_validation.json`.