Extract anonymous formatting functions from add_grade_column()

The `modify_table_body()` call inside `add_grade_column()` uses anonymous functions for label formatting (grade labels, "Any Grade" rows, indent logic). These could be extracted into named internal helpers for reuse and testability.

Ref: review comment on #228.

Start at the add_grade_column() entry point and inspect its modify_table_body() call, focusing on the anonymous functions for grade labels, "Any Grade" rows, and indentation. Trace nearby table-formatting code and existing tests if present; done means those behaviors are provided by named internal helpers without changing the output, with reusable helpers covered by tests.

Written by the indexing model from the issue text.