Embed validation-fitted calibration in BU risk heads

Replace the two inference-only BU risk heads with v2 releases. Each release embeds its matching five-horizon Platt calibration, fitted on known validation outcomes only; no test outcome was used for fitting.

risk_checkpoints/BU_S1_KD_S2/Mammo-FM_BU_S1_KD_S2_head.pt

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:6992aa888affb61b8cd897b1ee9e17fae8ef54bf0cb41f38d8935e63675c8273
-size 23381538
+oid sha256:01d6e2a095f16c30c940dd04738c18ca522d7e0f16919f4a35f858a1c24cdd38
+size 23384136

risk_checkpoints/BU_S1_KD_S2/Mammo-FM_BU_S2_only_head.pt

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:cf3e602acf5b0269ca03c7f851cad5d9b933d4a11d17b84a7b334f431a93fa9c
-size 23381474
+oid sha256:dc39c71342e8ee1cb77dbaee23119891bb9ae238ebb786f8ca9aa3014ef4d9db
+size 23384073