Reduce code duplication across codebase

## Summary

SonarCloud analysis shows 5.0% duplicated lines on new code. This technical debt should be addressed to improve maintainability.

## Duplication by Directory

| Directory | Duplication % | Duplicated Lines |
|-----------|---------------|------------------|
| OpenTDFKit | 4.4% | 308 |
| OpenTDFKitCLI | 6.8% | 124 |
| xtest | 14.1% | 34 |
| OpenTDFKitProfiler | 0.0% | 0 |
| .github/workflows | 0.0% | 0 |
| Package.swift | 0.0% | 0 |

## Areas to investigate

1. **OpenTDFKit (308 lines)** - Likely candidates:
- Crypto operations across TDF/NanoTDF implementations
- Key derivation code in KeyStore/CryptoHelper
- Similar encryption/decryption patterns

2. **OpenTDFKitCLI (124 lines)** - Likely candidates:
- Command handling patterns
- Environment variable parsing
- File I/O operations

3. **xtest (34 lines)** - Lower priority, test/integration code

## Suggested approach

- [ ] Run SonarCloud locally to identify specific duplicated blocks
- [ ] Extract common crypto patterns into shared utilities
- [ ] Create protocol extensions for repeated patterns
- [ ] Consider builder/factory patterns for similar object construction

## Acceptance criteria

- Reduce overall duplication to < 3%
- No duplication in new code contributions
- Maintain or improve test coverage

Run SonarCloud locally to identify the specific duplicated blocks in OpenTDFKit, OpenTDFKitCLI, and xtest. Review the reported crypto, key-derivation, command-handling, environment-parsing, and file-I/O patterns before choosing shared utilities or protocol extensions. Done means overall duplication is below 3%, new contributions add no duplication, and test coverage is maintained or improved.

Written by the indexing model from the issue text.