♻️ 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/obikpartitionner/src/partition.rs

@@ -186,7 +186,13 @@ impl KmerPartition {
     pub fn dereplicate(&self) -> SKResult<()> {
         let level = self.level;
         let root = &self.root_path;
-        let available = System::new_all().available_memory();
+        let sys = System::new_all();
+        // available_memory() can return 0 on macOS when the compressor page count exceeds
+        // free+inactive+purgeable pages (sysinfo saturating_sub). Fall back to half of total.
+        let available = match sys.available_memory() {
+            0 => sys.total_memory() / 2,
+            n => n,
+        };
         let n_threads = rayon::current_num_threads().max(1) as u64;
         let available_per_thread = available / n_threads;