refactor: streamline merge pipeline and MPHF indexing

Replace mphf.find() with direct mphf.index() calls to eliminate absence checks and fallback vectors. Introduce a lightweight MphfOnly wrapper for faster index loading, and standardize k-mer iteration across merge and rebuild layers. Update IndexMeta configuration and n_new calculation to leverage MPHF cardinality, streamlining the overall merge pipeline.

src/obilayeredmap/src/lib.rs

@@ -12,4 +12,4 @@ pub use layer::{Hit, Layer, LayerData};
 pub use layered_store::LayeredStore;
 pub use map::LayeredMap;
 pub use meta::{IndexMode, PartitionMeta};
-pub use mphf_layer::MphfLayer;
+pub use mphf_layer::{MphfLayer, MphfOnly};

src/obilayeredmap/src/mphf_layer.rs

@@ -129,7 +129,31 @@ impl MphfLayer {
     }
 
     pub fn n(&self) -> usize { self.n }
+}
 
+// ── MphfOnly ──────────────────────────────────────────────────────────────────
+
+/// Lightweight wrapper that loads only the MPHF file, without evidence or unitigs.
+///
+/// Use this when the caller guarantees that all queried kmers are in the MPHF
+/// domain (e.g. when iterating the source's own unitigs during merge).
+pub struct MphfOnly(Mphf);
+
+impl MphfOnly {
+    pub fn open(dir: &Path) -> OLMResult<Self> {
+        let mphf: Mphf = Mphf::load_full(&dir.join(MPHF_FILE))
+            .map_err(|e| OLMError::InvalidLayer(e.to_string()))?;
+        Ok(Self(mphf))
+    }
+
+    /// Return the slot for `kmer`. Only valid when `kmer` is in the MPHF domain.
+    #[inline]
+    pub fn index(&self, kmer: CanonicalKmer) -> usize {
+        self.0.index(&kmer.raw())
+    }
+}
+
+impl MphfLayer {
     // ── Build helpers ─────────────────────────────────────────────────────────
 
     pub fn unitig_writer(dir: &Path) -> OLMResult<UnitigFileWriter> {