feat: add optimized new_ones constructor for all-ones bit vectors

Introduces `new_ones` and `add_col_ones` methods to directly initialize all-ones bit vectors and matrix columns. This replaces redundant initialization sequences that created zero-filled structures and applied bitwise NOT, with a single pass that writes contiguous 0xFF bytes to disk. The change eliminates inversion overhead, streamlines test setup, and improves performance for filter mask intersection logic while preserving identical semantics.

src/obilayeredmap/src/layer.rs

@@ -106,9 +106,7 @@ impl Layer<()> {
         let presence_dir = layer_dir.join(PRESENCE_DIR);
         fs::create_dir_all(&presence_dir).map_err(OLMError::Io)?;
         let mut mb = PersistentBitMatrixBuilder::new(n_kmers, &presence_dir).map_err(OLMError::Io)?;
-        let mut col = mb.add_col().map_err(OLMError::Io)?;
-        col.not();
-        col.close().map_err(OLMError::Io)?;
+        mb.add_col_ones().map_err(OLMError::Io)?.close().map_err(OLMError::Io)?;
         mb.close().map_err(OLMError::Io)
     }
 }