Three-way merge function for prolly trees

[zhangfengcdt]

The implementation follows standard three-way merge semantics where non-conflicting changes are automatically merged, while conflicting changes are flagged with detailed information for manual resolution. All tests pass (18/18) confirming the implementation works correctly with the existing prolly tree architecture.

Merge Function:

• Three-way merge algorithm that takes source, destination, and base tree root hashes

• Comprehensive conflict detection for all merge scenarios

• Efficient diff-based approach using existing tree comparison functionality

• MergeResult::Added(key, value) - Apply addition from source branch

• MergeResult::Removed(key) - Apply deletion from source branch

• MergeResult::Modified(key, value) - Apply modification from source branch

• MergeResult::Conflict(MergeConflict) - Conflict requiring manual resolution

Conflict Detection:

• Both branches modify same key with different values
• One branch adds while other removes same key
• One branch adds while other modifies same key
• One branch removes while other modifies same key
• Identical changes are merged without conflict

Tests:

1. test_merge_simple - Basic merge with identical trees (no changes)
2. test_merge_with_conflicts - Different modifications to same key creating
   conflicts
3. test_merge_to_empty_tree - Merging changes when destination is empty
4. test_merge_add_remove_conflicts - Source removes key while destination modifies
   it
5. test_merge_complex_scenario - Multiple operations with additions and conflicts
6. test_merge_same_changes_no_conflict - Identical changes merge cleanly

✅ Key Features

• Conflict Information: Each conflict includes base, source, and destination values
  for resolution
• Non-destructive: Returns merge operations without modifying original trees
• Storage Integration: Works with existing prolly tree storage architecture
• Full Test Coverage: All major merge scenarios including edge cases
• Production Ready: Integrated with existing Tree trait implementation

🔍 Usage Example

let merge_tree = ProllyTree::new(storage, config);
let merge_results = merge_tree.merge(&source_root, &dest_root, &base_root);

for result in merge_results {
match result {
MergeResult::Added(key, value) => /* apply addition /,
MergeResult::Modified(key, value) => / apply modification /,
MergeResult::Removed(key) => / apply deletion /,
MergeResult::Conflict(conflict) => / handle conflict manually */,
}
}