split_tab = TRUE in tab_multi_manager is not working well

To reproduce the error :

```
library(rtauargus)
loc_tauargus <- "Y:/Logiciels/TauArgus/TauArgus4.2.3/TauArgus.exe"
options(rtauargus.tauargus_exe = loc_tauargus)

data("datatest1")
data("datatest2")

res <- tab_multi_manager(
list_tables = list(d1 = datatest1, d2 = datatest2),
list_explanatory_vars = list(
d1 = names(datatest1)[1:4],
d2 = names(datatest2)[1:5]
),
dir_name = "tauargus_files",
value = "pizzas_tot_abs",
freq = "nb_obs_rnd",
secret_var = "is_secret_prim",
totcode = "Total",
split_tab = TRUE
)
```

During the process of tab_multi_manager, it's important that the "primary secret" variable at each step is moving from one iteration to the following one.

Start by running the supplied R reproduction with tab_multi_manager(..., split_tab = TRUE) and trace how the split_tab path handles the primary secret variable across iterations. Confirm the variable is carried from one iteration to the next, and verify the reproduced output no longer shows the reported problem.

Written by the indexing model from the issue text.