Spacing in header is wrong when wrapping occurs if there are nonnested facets

The logic which handled spacing other elements of a header when a label needs word wrapping ends up assuming the column space is a single nested structure. We can see the consequence of that like so:

adsl <- ex_adsl
adsl$long_strata_lbls <- paste("really long strata label ", adsl$STRATA1)

lyt <- basic_table() |>
    split_cols_by("ARM") |>
    split_cols_by("SEX", split_fun = keep_split_levels(c("F", "M")), show_colcounts = TRUE) |>
    split_cols_by("STRATA1", nested = FALSE, labels_var = "long_strata_lbls") |>
    analyze("AGE")

tbl <- build_table(lyt, adsl)

cwds <- propose_column_widths(tbl)
cwds[8:10] <- 12

When we export the table with those column widths we get:

> cat(export_as_txt(tbl, colwidths = cwds))
          A: Drug X        B: Placebo       C: Combination                                               
         F        M        F        M         F         M                                                
                                                                  really         really         really   
                                                               long strata    long strata    long strata 
       (N=79)   (N=51)   (N=77)   (N=55)   (N=66)    (N=60)      label  A       label  B       label  C  
—————————————————————————————————————————————————————————————————————————————————————————————————————————
Mean   32.76    35.57    34.12    37.44     35.20     35.38       34.19          35.36          35.01    

Which is obviously not correct. This will be a tricky one to fix but it should be doable.