selected_100: restore full schema (gd_* / first_correct columns)

selected_100: purge MCQ + confirmed math_verify-mis-scored, re-band on audit-corrected K=16 (xVerify audit)

unsolved_100: drop 18 math_verify-mis-scored/MCQ problems, replace with double-verified-unsolved (xVerify audit)

coverage@K: 100 base-unsolved@K16 problems (0/16 correct) manifest

coverage@K: balanced 100 DeepMath problems (20/base-band) manifest