refactor: switch indexing to IndexMode and update metadata

Replace EvidenceKind with IndexMode (Exact, Approx, Hybrid) across layer construction and query dispatch. Update PartitionMeta and LayerMeta serialization to centralize index-wide configuration. Add flexible push_layer overloads to LayeredMap for dynamic index expansion without full rebuilds. Improve UnitigFileReader to gracefully fallback to sequential scanning when indexes are missing, eliminating panics.

src/obikpartitionner/src/rebuild_layer.rs

@@ -8,7 +8,7 @@ use obicompactvec::{PersistentBitMatrixBuilder,
                     PersistentCompactIntVecBuilder};
 use obidebruinj::GraphDeBruijn;
 use obiskio::{SKError, SKResult, UnitigFileReader};
-use obilayeredmap::{EvidenceKind, Layer, MphfLayer, OLMError};
+use obilayeredmap::{IndexMode, Layer, MphfLayer, OLMError};
 use obilayeredmap::meta::PartitionMeta;
 
 use crate::filter::KmerFilter;