Constructs a broker barrier from a settlement asset and bounds.
lowerBound and upperBound each accept a Pnl value object, a
DecimalInput, or null/undefined. At least one bound must be set;
the core validates this on build.
Throws AssetError when settlementAsset is empty or ParamError on
an invalid bound.
Broker-wide P&L-bounds barrier (at least one bound required).