(feat): refactor superkmer to use obipipeline with flat transforms

- Replace crossbeam-channel-based threading model
- Introduce obipipeline crate with Stage::Transform/Flat support  
- Replace single input + format detection by multiple inputs via PathIter
- Implement pipeline stages: open_chunks → normalize → build_superkmers (flat) + write_batch
- Add SharedFlatFn for 1→N transformations with delta tracking in scheduler loop

src/Cargo.lock

@@ -668,10 +668,10 @@ name = "obikmer"
 version = "0.1.0"
 dependencies = [
  "clap",
- "crossbeam-channel",
  "obifastwrite",
  "obikrope",
  "obikseq",
+ "obipipeline",
  "obiread",
  "obiskbuilder",
 ]