Replace BU risk folds with new S1-to-S2 and S2-only heads

Replaces the five historical fold checkpoints and manifest with the two new BU pretraining-exam-holdout risk heads. The S1-to-S2 head is the default release. Both heads use the frozen Mammo-FM_ASU_Trained_CLIP.tar encoder and produce raw, uncalibrated sigmoid scores. Their evaluation split is exact-exam-disjoint but not patient-disjoint.

risk_checkpoints/BU_S1_KD_S2/{fold1/Mammo-FM_BU_S1_KD_S2_fold1.pth-best.tar → Mammo-FM_BU_S1_KD_S2_head.pt}

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:f57730d674716c5870e5f0678c82c2a21cdff37eea518059f09809032830653e
-size 184831762
+oid sha256:6992aa888affb61b8cd897b1ee9e17fae8ef54bf0cb41f38d8935e63675c8273
+size 23381538

risk_checkpoints/BU_S1_KD_S2/{fold2/Mammo-FM_BU_S1_KD_S2_fold2.pth-best.tar → Mammo-FM_BU_S2_only_head.pt}

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:cac0884effa796de7fc32ac98b630301755d1d72a12a52addab24853a2c618a9
-size 184831762
+oid sha256:cf3e602acf5b0269ca03c7f851cad5d9b933d4a11d17b84a7b334f431a93fa9c
+size 23381474

risk_checkpoints/BU_S1_KD_S2/fold3/Mammo-FM_BU_S1_KD_S2_fold3.pth-best.tar

@@ -1,3 +0,0 @@
-version https://git-lfs.github.com/spec/v1
-oid sha256:f6b1cf5845c708a530ef8f5f98ecf2db87b54217f74b430e9b068ec99a40d1d6
-size 184831762

risk_checkpoints/BU_S1_KD_S2/fold4/Mammo-FM_BU_S1_KD_S2_fold4.pth-best.tar

@@ -1,3 +0,0 @@
-version https://git-lfs.github.com/spec/v1
-oid sha256:b1bf8769f3d6ee86253e36a473047396fb2e9d34470c440b8c5c4421dcbb9742
-size 184831762

risk_checkpoints/BU_S1_KD_S2/fold5/Mammo-FM_BU_S1_KD_S2_fold5.pth-best.tar

@@ -1,3 +0,0 @@
-version https://git-lfs.github.com/spec/v1
-oid sha256:0df00131c5a91ccd2e9d464bdbfb561f9604d589e38e61fa2903de5c50b8490f
-size 184831762

risk_checkpoints/BU_S1_KD_S2/manifest.json

@@ -1,46 +0,0 @@
-{
-  "schema_version": 1,
-  "model": "Mammo-FM BU S1 KD + S2",
-  "training_variant": "from_stage1",
-  "description": "Five final Stage-2 risk models initialized from Stage-1 knowledge distillation. Each model is evaluated only on its matching held-out BU patient fold.",
-  "outputs": "Five raw independent sigmoid probabilities; uncalibrated.",
-  "metric_aggregation": "Unweighted mean across five held-out folds; AUPRC uses average precision and year-specific eligibility masks.",
-  "files": [
-    {
-      "fold": 1,
-      "path": "risk_checkpoints/BU_S1_KD_S2/fold1/Mammo-FM_BU_S1_KD_S2_fold1.pth-best.tar",
-      "sha256": "f57730d674716c5870e5f0678c82c2a21cdff37eea518059f09809032830653e",
-      "bytes": 184831762,
-      "stored_epoch": 2
-    },
-    {
-      "fold": 2,
-      "path": "risk_checkpoints/BU_S1_KD_S2/fold2/Mammo-FM_BU_S1_KD_S2_fold2.pth-best.tar",
-      "sha256": "cac0884effa796de7fc32ac98b630301755d1d72a12a52addab24853a2c618a9",
-      "bytes": 184831762,
-      "stored_epoch": 4
-    },
-    {
-      "fold": 3,
-      "path": "risk_checkpoints/BU_S1_KD_S2/fold3/Mammo-FM_BU_S1_KD_S2_fold3.pth-best.tar",
-      "sha256": "f6b1cf5845c708a530ef8f5f98ecf2db87b54217f74b430e9b068ec99a40d1d6",
-      "bytes": 184831762,
-      "stored_epoch": 6
-    },
-    {
-      "fold": 4,
-      "path": "risk_checkpoints/BU_S1_KD_S2/fold4/Mammo-FM_BU_S1_KD_S2_fold4.pth-best.tar",
-      "sha256": "b1bf8769f3d6ee86253e36a473047396fb2e9d34470c440b8c5c4421dcbb9742",
-      "bytes": 184831762,
-      "stored_epoch": 6
-    },
-    {
-      "fold": 5,
-      "path": "risk_checkpoints/BU_S1_KD_S2/fold5/Mammo-FM_BU_S1_KD_S2_fold5.pth-best.tar",
-      "sha256": "0df00131c5a91ccd2e9d464bdbfb561f9604d589e38e61fa2903de5c50b8490f",
-      "bytes": 184831762,
-      "stored_epoch": 5
-    }
-  ],
-  "validation_status": "Historical benchmark checkpoints identified and file hashes verified; fresh port inference validation is separate and pending."
-}