[bug] Choice letter: （B）, \boxed{B.}, and 答案是B never extract (MMLU FN)

[marktech0813]

Problem

Additional common MCQ finals still extract as None (beyond #491):

extract_choice_letter("（B）")       # None  (fullwidth parens)
extract_choice_letter(r"\boxed{B.}") # None  (trailing period inside box)
extract_choice_letter("答案是B")      # None  (Chinese "answer is")
score_text("mmlu", "（B）", "B")           # 0.0
score_text("mmlu", r"\boxed{B.}", "B")    # 0.0
score_text("mmlu", "答案是B", "B")          # 0.0

Expected

All three extract B and score 1.0.

Environment

• origin/main