Extend mergeQuerySettings() to support per-knob merge for the queryKnobs sub-document. Today, setQuerySettings replaces settings wholesale; with query knobs, callers need to add, update, or remove individual knobs without clobbering the rest. null on the wire acts as a removal sentinel (stored as std::monostate, consumed during merge, stripped before persistence). If all knobs are removed, the queryKnobs field is dropped entirely. No value validation here; that is wired in the validation milestone.