feat: support exact and approximate evidence in layer construction

Refactored `MphfLayer::build` to accept an `EvidenceKind` parameter, routing to exact (index-based, parallel MPHF, writes `evidence.bin`) or approximate (sequential mmap iterator, writes `fingerprint.bin`) pipelines. Introduced `CanonicalKmerIter` for memory-mapped, chunked k-mer iteration with O(1) resets via `Arc<Mmap>`. Updated layer and map APIs to forward evidence kind, added `push_layer` for count matrices, and adjusted tests and public exports accordingly.

src/obikpartitionner/src/rebuild_layer.rs

@@ -8,7 +8,7 @@ use obicompactvec::{PersistentBitMatrixBuilder,
                     PersistentCompactIntVecBuilder};
 use obidebruinj::GraphDeBruijn;
 use obiskio::{SKError, SKResult, UnitigFileReader};
-use obilayeredmap::{Layer, MphfLayer, OLMError};
+use obilayeredmap::{EvidenceKind, Layer, MphfLayer, OLMError};
 use obilayeredmap::meta::PartitionMeta;
 
 use crate::filter::KmerFilter;
@@ -146,7 +146,7 @@ impl KmerPartition {
         uw.close()?;
         drop(g);
 
-        Layer::<()>::build(&dst_layer_dir, block_bits).map_err(olm_to_sk)?;
+        Layer::<()>::build(&dst_layer_dir, block_bits, &EvidenceKind::Exact).map_err(olm_to_sk)?;
         let dst_mphf = MphfLayer::open(&dst_layer_dir).map_err(olm_to_sk)?;
 
         // ── Prepare matrix builders (one column per genome) ───────────────────