Remove MendeleyCXR (no longer part of the FleXray evaluation) and update README

Card: FluXray now ships in this repo; licenses listed per folder

Redistribute BTXRD (CC BY 4.0, figshare 27865398) in the fxr-dataset format

Add per-dataset split assignments and quality-control exclusions

Dataset card: describe FluXray inline

Dataset card: describe the FluXray companion release files

refine dataset citation and availability notes

update links and citation guidance

Add FleXray data release: 7 repackaged X-ray sources + MURA forearm/humerus annotations