feat: add configurable block sizes and in-place reindex command

Propagate configurable block size (`block_bits`) through index and layer construction to control unitig chunking and optimize memory/performance trade-offs. Introduce an in-place `reindex` command and library method to convert indices between exact and approximate evidence formats. Add validation to reject merging indexes with mismatched evidence types, and update parallel kmer counting to use `AtomicUsize` for thread-safe aggregation. Includes CLI argument parsing, metadata persistence, and updated tests.

src/obikpartitionner/src/rebuild_layer.rs

@@ -96,6 +96,7 @@ impl KmerPartition {
         filters: &[Box<dyn KmerFilter>],
         mode: MergeMode,
         n_genomes: usize,
+        block_bits: u8,
     ) -> SKResult<()> {
         let src_index_dir = src.part_dir(i).join(INDEX_SUBDIR);
         if !src_index_dir.exists() {
@@ -145,7 +146,7 @@ impl KmerPartition {
         uw.close()?;
         drop(g);
 
-        Layer::<()>::build(&dst_layer_dir).map_err(olm_to_sk)?;
+        Layer::<()>::build(&dst_layer_dir, block_bits).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) ───────────────────