Add cross-implementation consistency tests for dataset hash functionality:
## Test Fixtures
- Single file, nested directories, and multiple file test cases
- Expected hashes in JSON format for validation
## Test Infrastructure
- harness.go: Common test utilities and reference implementation runner
- dataset_hash_test.go: Consistency test cases comparing implementations
- cmd/update.go: Tool to regenerate expected hashes from reference
## Purpose
Ensures hash implementations (Go, C++, Zig) produce identical results
across all supported platforms and implementations.