feat: add fixed-pool scoliosis partition helper

datasets/Scoliosis1K/README.md

@@ -69,6 +69,33 @@ python -m torch.distributed.launch --nproc_per_node=4 \
 opengait/main.py --cfgs configs/sconet/sconet_scoliosis1k.yaml --phase test --log_to_file
 ```
 
+### Fixed-pool ratio comparison
+
+If you want to compare `1:1:2` against `1:1:8` without changing the evaluation
+pool, do not compare `Scoliosis1K_112.json` against `Scoliosis1K_118.json`
+directly. Those two files differ substantially in train/test membership.
+
+For a cleaner same-pool comparison, use:
+
+* `datasets/Scoliosis1K/Scoliosis1K_118.json`
+  * original `1:1:8` split
+* `datasets/Scoliosis1K/Scoliosis1K_118_fixedpool_train112.json`
+  * same `TEST_SET` as `118`
+  * same positive/neutral `TRAIN_SET` ids as `118`
+  * downsampled `TRAIN_SET` negatives to `148`, giving train counts
+    `74 positive / 74 neutral / 148 negative`
+
+The helper used to generate that derived partition is:
+
+```bash
+uv run python scripts/build_scoliosis_fixedpool_partition.py \
+  --base-partition datasets/Scoliosis1K/Scoliosis1K_118.json \
+  --dataset-root /mnt/public/data/Scoliosis1K/Scoliosis1K-sil-pkl \
+  --negative-multiplier 2 \
+  --output-path datasets/Scoliosis1K/Scoliosis1K_118_fixedpool_train112.json \
+  --seed 118
+```
+
 ### Modality sanity check
 
 The silhouette and skeleton-map pipelines are different experiments and should not be mixed when you interpret results.