Add complete B+ tree redistribution and Vec-compatible interface to CompressedLeafNode

- Implement borrowing methods for node redistribution:
  - borrow_last(): Remove and return last key-value pair for left sibling donation
  - borrow_first(): Remove and return first key-value pair for right sibling donation
  - Both methods respect can_donate() constraints and maintain sorted order

- Implement acceptance methods for receiving redistributed items:
  - accept_from_left(): Insert item at beginning from left sibling
  - accept_from_right(): Insert item at end from right sibling
  - Both maintain internal sorted order through proper insertion

- Implement merge_from() for node consolidation:
  - Efficiently merge all content from another node using ptr::copy_nonoverlapping
  - Preserve next pointer chain for linked list integrity
  - Clear source node after merge

- Add Vec-compatible interface with wrapper types:
  - CompressedKeyView: Read-only Vec-like access to keys with indexing
  - CompressedValueView: Read-only Vec-like access to values with indexing
  - CompressedValueViewMut: Mutable Vec-like access to values with indexing
  - All support len(), is_empty(), get(), and Index/IndexMut traits

- Add comprehensive test coverage (50 total tests):
  - Borrowing scenarios: basic functionality, donation constraints, edge cases
  - Redistribution simulation: realistic left-to-right donation scenario
  - Merging functionality: content consolidation and next pointer handling
  - Vec interface: indexing, bounds checking, mutable operations, panic behavior

- All tests pass: 50 CompressedLeafNode tests + 81 library tests + 81 integration tests
- Maintains 256-byte (4 cache lines) memory footprint
- Full API compatibility with LeafNode for B+ tree operations

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

Author: KentBeck