✨ add PhantomData import for generic type safety

- Added `use std::marker::PhantomData;` to prepare for generic scheduler implementations
- Ensures type safety and avoids unused lifetime/type parameters warnings

src/obikrope/src/cursor.rs

@@ -27,7 +27,7 @@
 //! ```
 //! use obikrope::{Rope, RopeCursor};
 //!
-//! let mut rope = Rope::new();
+//! let mut rope = Rope::new(None);
 //! rope.push(b"ACGT".to_vec());
 //!
 //! let cursor = rope.fw_cursor();