Update Square purpose-layout checkpoints (full wave + d1/d2/e2 complete)

Remove stray HF_ROOT_README.md

Remove obsolete dated prefix square_r1_20260723/ (contaminated or superseded)

Remove obsolete dated prefix square20_20260722/ (contaminated or superseded)

Reorganize by purpose (family/data/split); drop dated prefixes

Add square_r1_20260723 finished ckpts + update model card

Add Square 20% wave ckpts 20260722 (square20_20260722/ + BC sb4)

initial commit