♻️ 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/obiread/Cargo.toml

@@ -4,6 +4,6 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
-bytes  = "1"
+obikrope = { path = "../obikrope" }
 niffler = { version = "2", default-features = false, features = ["gz", "bz2", "lzma", "zstd"] }
 ureq   = "2"