009a328c5818c781b0a3ce93608584c8fbafebbe
Introduce a secondary layer iteration to open `SrcLayerData` alongside the unitig reader for concurrent metadata access. This refactors the merge routine to handle kmer deduplication and per-layer data initialization simultaneously. Also corrects a typo in `rebuild_layer.rs` where `openopen_sequential` is renamed to `open_sequential`.