Add hashing verification and local test harness

[pdj555]

Summary

• add hash functions and previous-hash tracking in blocks
• verify blockchain integrity with verifyChain and network-wide with verifyAllChains
• implement a tiny in-repo gtest replacement and adjust tests to use it
• fix ctest invocation in CI workflow

Testing

• cmake -S . -B build
• cmake --build build
• ctest --test-dir build --output-on-failure