chore: make COUNTS_DIR constant pub(crate)

Adjusts the visibility of the COUNTS_DIR constant to pub(crate), enabling internal crate-wide access without altering runtime behavior or data structures.

src/obilayeredmap/src/layer.rs

@@ -16,7 +16,7 @@ use crate::meta::IndexMode;
 use crate::mphf_layer::MphfLayer;
 pub(crate) use crate::mphf_layer::UNITIGS_FILE;
 
-const COUNTS_DIR:   &str = "counts";
+pub(crate) const COUNTS_DIR:   &str = "counts";
 const PRESENCE_DIR: &str = "presence";
 
 // ── Trait ─────────────────────────────────────────────────────────────────────