Implement the QuerySettingsKnobOverlay C++ class: fromBSON() deserializes a queryKnobs BSON object into pre-parsed index/value pairs using QueryKnobRegistry for wire-name resolution; toBSON() reconstructs BSON via index-to-wire-name reverse lookup. Pure data type with no hash, merge, or feature flag logic. fromBSON() rejects unknown knob names. Hash, IDL wiring, and feature flag gating are handled by follow-up tickets.