♻️ 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

Betula_exilis--IGA-24-33/partition.meta

@@ -2,6 +2,5 @@
   "n_bits": 8,
   "kmer_size": 31,
   "minimizer_size": 11,
-  "format": "zstd",
   "level": 3
 }