Function build_in_memory_sample_sort 

pub fn build_in_memory_sample_sort<S, F>(
    text: &[S],
    opts: &ExtMemOpts,
    emit: F,
) -> Result<()>
where
    S: Symbol,
    F: FnMut(u64) -> Result<()>,

In-memory variant of the sample-sort algorithm used by build_ext_mem. Skips all disk I/O at the cost of holding the (pos, lcp) records in RAM throughout. Picks u32 records when n ≤ 2³², falls back to u64 otherwise. The caller’s emit closure is called once per output position in lex order, just like in the ext-mem path.