Implement complete CompressedLeafNode functionality with proper next field linking

- Add remove() method with underfull detection matching LeafNode behavior
- Implement iterator support with CompressedLeafIter for sorted traversal
- Add proper next field linking during splits:
  - Right node inherits original next pointer
  - Left node gets NULL_NODE for caller to update
  - Preserves linked list integrity during node splits
- Add comprehensive next field accessor methods (next(), set_next())
- Update to_leaf_node() to preserve next field when converting
- Add extensive test coverage:
  - Remove operations: single/multiple items, edge cases, underfull detection
  - Iterator functionality: empty, single, multiple items, boundary values
  - Next field linking: basic ops, split preservation, conversion integrity
- All 37 CompressedLeafNode tests pass, maintaining API consistency with LeafNode
- Ensures proper B+ tree linked list maintenance during splits and merges

Co-authored-by: Ona <no-reply@ona.com>

Author: KentBeck