Function parse_main_pane_order 

pub fn parse_main_pane_order(value: &str) -> Option<[MainVerticalPane; 3]>

What: Parse main_pane_order value into a length-3 permutation of distinct roles.

Inputs:

• value: Comma-separated tokens (whitespace allowed).

Output:

• Some(order) when exactly three distinct known roles are present; otherwise None.

Details:

• Empty or duplicate roles yield None.