Implement self-registering QueryKnobBase/QueryKnob<T> descriptor infrastructure. Each descriptor carries a readGlobal function pointer via a concept-dispatched readGlobalValue<> template (AtomicLoadable for AtomicWord<T>, SynchronizedGettable for synchronized_value<Enum>), and a fromBSON deserializer auto-set by the constructor. QueryKnobBase::all() collects all descriptors at static init for the registry to consume in M3. Unit tests use synthetic server parameters to verify self-registration, readGlobal, and fromBSON behavior.