refactor: extract matrix helpers and improve bit iteration ergonomics

Refactor parallel matrix construction by extracting reusable `pairwise_matrix` and `pairwise2_matrix` helpers, and consolidate binary record deserialization into dedicated parsing functions. Add `set` and `iter` methods to `BitSliceMut` and `MemoryBitVec` for ergonomic bit manipulation and iteration. Standardize JSON field extraction via `meta::field`, expose `MemoryBitIter`, and improve test reliability by automatically cleaning up temporary directories.

src/obilayeredmap/src/layer.rs

@@ -6,6 +6,7 @@ use obicompactvec::{
     PersistentBitMatrix, PersistentBitMatrixBuilder,
     PersistentCompactIntMatrix, PersistentCompactIntMatrixBuilder,
 };
+use obicompactvec::traits::BitSliceMut;
 use obikseq::CanonicalKmer;
 use obiskio::{UnitigFileReader, UnitigFileWriter};
 

src/obilayeredmap/src/layered_store.rs

@@ -102,6 +102,7 @@ mod tests {
         PersistentBitMatrix, PersistentBitMatrixBuilder,
         PersistentCompactIntMatrix, PersistentCompactIntMatrixBuilder,
     };
+    use obicompactvec::traits::BitSliceMut;
     use tempfile::tempdir;
 
     fn make_int_matrix(cols: &[&[u32]]) -> (tempfile::TempDir, PersistentCompactIntMatrix) {