feat: enforce canonical k-mer representation throughout the codebase

Refactor core types to consistently use `CanonicalKMer` (lexicographically minimal of k-mer and its reverse complement) as the canonical representation, ensuring deterministic behavior in graph traversal (unitig decomposition), neighbor resolution (`unique_neighbor` with `[CanonicalKmer; 4]` input) and scatter output generation. Introduce `RoutableSuperKmer`, add `.seq_hash()` support, fix type syntax errors in unitig extraction methods and deduplication tests. Update all k-mer construction to use canonical-aware APIs, including unsafe unchecked constructors for performance-critical paths.

src/obikpartitionner/src/partition.rs

@@ -5,7 +5,7 @@ use std::path::{Path, PathBuf};
 use tracing::{debug, info};
 
 use memmap2::MmapMut;
-use obikseq::kmer::Kmer;
+use obikseq::kmer::CanonicalKmer;
 use ph::fmph::GOFunction;
 
 use sysinfo::System;
@@ -509,7 +509,7 @@ fn count_partition(dir: &Path, dedup_path: &Path, k: usize) -> SKResult<()> {
     debug!("{}: sidecar capacity estimate={capacity}", dir.display());
 
     // Pass 1: collect all unique canonical kmers.
-    let mut seen: HashSet<Kmer> = HashSet::with_capacity(capacity);
+    let mut seen: HashSet<CanonicalKmer> = HashSet::with_capacity(capacity);
     let mut pass1_superkmers: u64 = 0;
     {
         let mut reader = SKFileReader::open(dedup_path, k)?;
@@ -520,7 +520,7 @@ fn count_partition(dir: &Path, dedup_path: &Path, k: usize) -> SKResult<()> {
             }
         }
     }
-    let kmers: Vec<Kmer> = seen.into_iter().collect();
+    let kmers: Vec<CanonicalKmer> = seen.into_iter().collect();
     let n_kmers = kmers.len();
     debug!(
         "{}: pass1 superkmers={pass1_superkmers} unique_kmers={n_kmers}",