♻️ 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/obiskbuilder/Cargo.toml

@@ -5,5 +5,4 @@ edition = "2024"
 
 [dependencies]
 obikseq = { path = "../obikseq" }
-obiread = { path = "../obiread" }
-bytes = "1"
+obikrope = { path = "../obikrope" }