Add model card: what this is, and which variants are valid

consolidate: drop the duplicated root copy

merged checkpoint ({'pair_id': 'beetle-humanscale-nld-eng', 'parent_a': 'Beetle-HumanScale/beetle-monolingual-humanscale-nld', 'parent_b': 'Beetle-HumanScale/beetle-monolingual-humanscale-eng', 'ceiling': 'Beetle-HumanScale/beetle-bilingual-l2-50-simultaneous-b2-humanscale-nld-eng', 'operator': 'average', 'alignment': 'naive', 'align_method': 'permutation', 'regime': 'shared_base', 'eval_langs': 'eng+nld', 'nll_merge': 7.6155, 'nll_floor': 6.3583, 'param_coverage': 1.0, 'MS': -4.1098})