♻️ refactor rope implementation to use obikrope

- rename `obirope` → `obikroper`
- replace legacy rope with new in-place, Cell-based implementation
  - add ForwardCursor/Backward Cursor & SeekMode support (no more BytesMut)
- update all dependents:
  - obiread: switch to Rope + cursors, remove tape.rs
    • chunk iterator yields `Rope` instead of Vec<Bytes>
  - obiskbuilder: use ForwardCursor over Rope
- remove bytes dependency from affected crates

src/Cargo.toml

@@ -1,3 +1,3 @@
 [workspace]
 resolver = "3"
-members = ["obikseq", "obiread", "obiskbuilder", "obifastwrite", "obikmer"]
+members = ["obikseq", "obiread", "obiskbuilder", "obifastwrite", "obikmer","obikrope"]