♻️ 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.lock

@@ -595,6 +595,14 @@ dependencies = [
  "obiskbuilder",
 ]
 
+[[package]]
+name = "obikrope"
+version = "0.1.0"
+dependencies = [
+ "bytes",
+ "criterion2",
+]
+
 [[package]]
 name = "obikseq"
 version = "0.1.0"
@@ -607,8 +615,8 @@ dependencies = [
 name = "obiread"
 version = "0.1.0"
 dependencies = [
- "bytes",
  "niffler",
+ "obikrope",
  "ureq",
 ]
 
@@ -616,9 +624,8 @@ dependencies = [
 name = "obiskbuilder"
 version = "0.1.0"
 dependencies = [
- "bytes",
+ "obikrope",
  "obikseq",
- "obiread",
 ]
 
 [[package]]