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/index_layer.rs

@@ -110,7 +110,7 @@ impl KmerPartition {
         uw.close()?;
 
         if with_counts {
-            Layer::<PersistentCompactIntMatrix>::build(&layer_dir, block_bits, |kmer| {
+            Layer::<PersistentCompactIntMatrix>::build(&layer_dir, block_bits, evidence, |kmer| {
                 match (&mphf1_opt, &counts1_opt) {
                     (Some(mphf), Some(counts)) => counts.get(mphf.index(&kmer.raw())),
                     _ => 1,
@@ -118,13 +118,7 @@ impl KmerPartition {
             })
             .map_err(olm_to_sk)?;
         } else {
-            Layer::<()>::build(&layer_dir, block_bits).map_err(olm_to_sk)?;
-        }
-
-        // For approximate evidence: replace the exact evidence bundle with a
-        // fingerprint.  For exact evidence, build() already wrote it.
-        if let EvidenceKind::Approx { b, z } = evidence {
-            Layer::<()>::build_approx_evidence(&layer_dir, *b, *z).map_err(olm_to_sk)?;
+            Layer::<()>::build(&layer_dir, block_bits, evidence).map_err(olm_to_sk)?;
         }
 
         // Write meta.json in the index/ directory so LayeredMap::open works

src/obikpartitionner/src/merge_layer.rs

@@ -9,7 +9,7 @@ use obicompactvec::{PersistentBitMatrix, PersistentBitMatrixBuilder,
                     PersistentCompactIntVecBuilder};
 use obikseq::CanonicalKmer;
 use obiskio::{SKError, SKResult, UnitigFileReader};
-use obilayeredmap::{Layer, LayeredMap, MphfLayer, OLMError};
+use obilayeredmap::{EvidenceKind, Layer, LayeredMap, MphfLayer, OLMError};
 use obilayeredmap::meta::PartitionMeta;
 
 use crate::partition::KmerPartition;
@@ -217,7 +217,7 @@ impl KmerPartition {
                 uw.write(&unitig)?;
             }
             uw.close()?;
-            Layer::<()>::build(&new_layer_dir, block_bits).map_err(olm_to_sk)?;
+            Layer::<()>::build(&new_layer_dir, block_bits, &EvidenceKind::Exact).map_err(olm_to_sk)?;
         }
         drop(g);
 

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) ───────────────────