Add dataset card for phase1_n20_hardware_25k

datasets/phase1_n20_hardware_25k/README.md

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+# Qubit Mapping Datasets
+
+Dataset repo path: `datasets/phase1_n20_hardware_25k`
+
+## Dataset
+
+- Name: `phase1_n20_hardware_25k`
+- Description: 25k hardware-only phase-1 mapping dataset at n=20
+- Total size (bytes): `271240434`
+- Uploaded at (UTC): `2026-03-30T07:00:35.866878+00:00`
+
+## Files
+
+- `config.json` (1222 bytes)
+- `data_summary.json` (180 bytes)
+- `target_quality_by_family.csv` (537 bytes)
+- `target_quality_overall.json` (140 bytes)
+- `test.pt` (32155385 bytes)
+- `train.pt` (206370171 bytes)
+- `val.pt` (32712799 bytes)
+
+## Download
+
+Use `snapshot_download` with the recorded allow-patterns in the local manifest.