Function check_proportional 

pub fn check_proportional(
    t1: usize,
    t2: usize,
    t4: usize,
    plane: usize,
) -> Option<Ratio>

2026-09-26: Check that temporal groups scale with duration, from the totals of three clips at 1x, 2x and 4x, and infer the group count and template overhead.

Two totals cannot test this: any difference t2 - t1 fits a 2:1 ratio once the implied overhead absorbs the rest. With three the system is over-determined:

  t4 - t2  ==  2 * (t2 - t1)

which involves neither the overhead nor the tokens-per-group figure. It also returns None when t2 - t1 is zero or not a whole number of planes, when plane is zero, when the implied overhead is negative, and on overflow.