Constructs an account-asset barrier from its parts.
initialPnl seeds the account's running P&L; lowerBound/upperBound
accept a Pnl, a DecimalInput, or null/undefined (at least one
bound required, validated on build).
Throws AssetError when settlementAsset is empty or ParamError on
an invalid value.
Per-(account, settlement-asset) P&L-bounds barrier.