♻️ refactor pipeline architecture and fix macOS memory detection

- Replace WorkerPool-based pipelines with typed `Pipe` abstraction in obipipeline
  - Introduce Pipe/PipeIter for composable, sourceless/sink-less pipelines
- Update partition and superkmer commands to use new Pipe API via make_pipe!
  - Remove Arc<Mutex<...>> patterns; simplify state management
- Fix macOS available_memory() returning 0 by falling back to half total memory in dereplicate()
- Remove unused `format: "zstd"` field from partition.meta

src/obikseq/src/kmer.rs

@@ -161,6 +161,33 @@ impl Kmer {
     pub fn hash(&self, k: usize) -> u64 {
         mix64(self.canonical(k).0)
     }
+
+    /// Return the left canonical neighbors of this kmer.
+    ///
+    /// Zero allocation — result lives on the stack.
+    pub fn left_canonical_neighbors(&self, k: usize) -> [Kmer; 4] {
+        let shifted = (self.0 >> 2) & (!0u64 << (64 - 2 * k));
+        [
+            Kmer(shifted).canonical(k),
+            Kmer(shifted | (1u64 << 62)).canonical(k),
+            Kmer(shifted | (2u64 << 62)).canonical(k),
+            Kmer(shifted | (3u64 << 62)).canonical(k),
+        ]
+    }
+
+    /// Return the right canonical neighbors of this kmer.
+    ///
+    /// Zero allocation — result lives on the stack.
+    pub fn right_canonical_neighbors(&self, k: usize) -> [Kmer; 4] {
+        let shifted = self.0 << 2 & (!0u64 << (64 - 2 * (k - 1)));
+        let shift = 64 - 2 * k;
+        [
+            Kmer(shifted).canonical(k),
+            Kmer(shifted | (1u64 << shift)).canonical(k),
+            Kmer(shifted | (2u64 << shift)).canonical(k),
+            Kmer(shifted | (3u64 << shift)).canonical(k),
+        ]
+    }
 }
 
 // ── tests ─────────────────────────────────────────────────────────────────────