value method
Returns the runtime value for fieldName.
This accessor is intentionally dynamic for calculated-field ergonomics,
so expressions like context.value('amount') ?? 0 keep working with
FdcDecimal arithmetic. During calculated-field evaluation, null decimal
field values are exposed as FdcDecimal.zero so mixed decimal expressions
do not accidentally dispatch through Dart's int/double operators.
Prefer typed helpers such as numValue, decimalValue, intValue, and
doubleValue when strict static typing is more important than expression
brevity.
Implementation
dynamic value(String fieldName) => valueAt(_indexOf(fieldName));