Adds examples/cpp_consumer.cpp + a 'make test-cpp' target proving a C++
translation unit links libluxconsensus statically and drives a block to
finality through the C ABI — no Go runtime, no gRPC, no zero-knowledge layer.
This is the seam for replacing an embedded Raft/Keeper (ZooKeeper API) with
leaderless Lux consensus *natively* in a C++ engine (e.g. the ClickHouse-derived
Hanzo Datastore's ReplicatedMergeTree coordination), rather than porting Go to
C++. Verified: 'make test-cpp' -> 'C++ reached consensus finality — zero Raft,
zero ZK'.
