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/obiskbuilder/src/iter.rs

@@ -16,6 +16,7 @@
 //! | super-kmer length = 256| k              |
 
 use obikrope::{ForwardCursor, Rope, RopeCursor};
+use obikseq::kmer::CanonicalKmer;
 use obikseq::RoutableSuperKmer;
 
 use crate::rolling_stat::RollingStat;
@@ -29,7 +30,7 @@ pub struct SuperKmerIter<'a> {
     theta: f64,
     scratch: SuperKmerScratch,
     stat: RollingStat,
-    prev_min: Option<u64>,
+    prev_min: Option<CanonicalKmer>,
     prev_min_pos: usize,
 }
 
@@ -107,7 +108,7 @@ impl Iterator for SuperKmerIter<'_> {
                 continue;
             }
 
-            let min = self.stat.canonical_minimizer_raw().unwrap_or(0);
+            let min = self.stat.canonical_minimizer().unwrap(); // always Some after ready()
             let min_pos = self.stat.minimizer_position().unwrap_or(0);
 
             // ── 2. Minimizer change check ─────────────────────────────────────

src/obiskbuilder/src/rolling_stat.rs

@@ -1,4 +1,4 @@
-use obikseq::kmer::Kmer;
+use obikseq::kmer::{CanonicalKmer, Kmer};
 
 use crate::encoding::encode_nuc;
 use crate::entropy_table::{WS_MAX, emax, entropy_norm_kmer, ln_class_size, log_nwords, n_log_n};
@@ -283,6 +283,12 @@ impl RollingStat {
         }
     }
 
+    pub fn canonical_minimizer(&self) -> Option<CanonicalKmer> {
+        self.canonical_minimizer_raw().map(|raw| {
+            CanonicalKmer::from_raw_unchecked(Kmer::from_raw_right(raw, self.m).raw())
+        })
+    }
+
     pub fn entropy(&self, order: usize) -> Option<f64> {
         if !self.ready() {
             return None;