## 1. Implementation
- [x] 1.1 Create _create_composite_key_array() helper function
- [x] 1.2 Create _filter_by_key_membership() helper using PyArrow join
- [x] 1.3 Update streaming deduplication to use Arrow Table for seen keys
- [x] 1.4 Update merge operations to use vectorized multi-key matching
- [x] 1.5 Add binary_join_element_wise fallback for heterogeneous types

## 2. Testing
- [x] 2.1 Test multi-column key deduplication performance
- [x] 2.2 Test merge operations with composite keys
- [x] 2.3 Test various data type combinations in composite keys
- [x] 2.4 Benchmark against single-column key performance

## 3. Documentation
- [x] 3.1 Document multi-column key performance characteristics
- [x] 3.2 Add examples for composite key usage
- [x] 3.3 Update API reference with supported key types
