refactor: improve resource cleanup and index packing

Explicitly close file handles and remove temporary artifacts after serialization to prevent disk space leaks. Additionally, compact internal matrix structures immediately upon loading the KmerIndex to improve memory efficiency and prepare for downstream operations.

src/obikindex/src/rebuild.rs

@@ -112,6 +112,8 @@ impl KmerIndex {
         // Write SENTINEL_INDEXED — output is ready to use.
         fs::File::create(output.join(SENTINEL_INDEXED))?;
 
-        KmerIndex::open(output)
+        let idx = KmerIndex::open(output)?;
+        idx.pack_matrices()?;
+        Ok(idx)
     }
 }