Extract dereplication logic into obikderep crate and centralize paths

Move partition dereplication logic into a dedicated `obikderep` crate that implements a two-phase hash-split/merge strategy with Rayon for parallel processing. Centralize superkmer file path construction in `obilayeredmap` and update dependent crates to use the new pipeline and shared path helpers. Adjust test suites to explicitly invoke the dereplication step.

src/obikindex/src/index_layer.rs

@@ -43,7 +43,7 @@ impl KmerIndex {
         block_bits: u8,
     ) -> Result<usize, SKError> {
         let layer0_dir = self.layer_dir(i, 0);
-        let dedup_path = layer0_dir.join("dereplicated.skmer.zst");
+        let dedup_path = obilayeredmap::dereplicated_superkmers_path(&layer0_dir);
         if !dedup_path.exists() {
             return Ok(0);
         }
@@ -123,7 +123,7 @@ impl KmerIndex {
     /// Deletes `dereplicated.skmer.zst` (+ sidecar), `mphf1.bin`, `counts1.bin`.
     pub fn remove_build_artifacts(&self, i: usize) {
         let layer0_dir = self.layer_dir(i, 0);
-        let dedup = layer0_dir.join("dereplicated.skmer.zst");
+        let dedup = obilayeredmap::dereplicated_superkmers_path(&layer0_dir);
         remove_if_exists(&SKFileMeta::sidecar_path(&dedup));
         remove_if_exists(&dedup);
         remove_if_exists(&layer0_dir.join("mphf1.bin"));