Implement header-only QueryKnobSnapshot: a runtime-sized vector container with typed get<T>/set accessors, per-slot source tracking (default, setParameter, PQS override), enum cast support, and value semantics. The type parameter T is provided by the caller (QueryKnobConfiguration forwards it from the QueryKnob<T> descriptor). Unit tests use synthetic indices to verify get/set round-trips for all variant types, enum casts, source tracking, and copy independence.