filters := &FilterSet{
    Must: &ConditionSet{
        Conditions: []FilterCondition{
            TextCondition{Key: "search_store_id", Value: "store-123"},
            TextAnyCondition{Key: "city", Values: []string{"London", "Berlin"}},
            TimeRangeCondition{
                Key:   "created_at",
                Value: TimeRange{Gte: &startTime, Lt: &endTime},
            },
            NumericRangeCondition{
                Key:       "price",
                Value:     NumericRange{Gte: &minPrice, Lte: &maxPrice},
                FieldType: UserField,
            },
        },
    },
    MustNot: &ConditionSet{
        Conditions: []FilterCondition{
            BoolCondition{Key: "deleted", Value: true},
            IsEmptyCondition{Key: "tags"},
        },
    },
}

{
  "must": [
    {"field": "search_store_id", "equalTo": "store-123"},
    {"field": "city", "anyOf": ["London", "Berlin"]},
    {"field": "created_at", "atOrAfter": "2024-01-01T00:00:00Z", "before": "2024-12-31T00:00:00Z"},
    {"field": "price", "greaterThanOrEqualTo": 100, "lessThanOrEqualTo": 500}
  ],
  "mustNot": [
    {"field": "deleted", "equalTo": true},
    {"field": "tags"}
  ]
}

payload := BuildPayload(
    map[string]any{"search_store_id": "store-123"},  // Internal (top-level)
    map[string]any{"document_id": "doc-456"},        // User → custom.document_id
)