use Maybe type for error handling (Fail case)

[unional]

NumericStruct.Add<NumericStruct.FromNumeric<A, Fail>, NumericStruct.FromNumeric<B, Fail>>
//              ^ fail as input not accepting `Fail`

Instead of:

type NumericStruct.Add<A extends NumericStruct, B extends NumericStruct> ...

Accept Maybe<NumericStruct> and propagate the error.

This will simplify many implementation.

[unional]

Another thing to consider is how to add rich error processing.

i.e. instead of just pass down and up the error, we can process it.

e.g.

type Add<A, B> = DoWork<A, B> extends infer Result 
  ? Result extends FailBrand<unknown>
    ? `Cannot add '${A}` and `${B}` due to ${Result.message}`
    : Result
  : never

The idea is to use branding for fail case when do a brand check to see if the result is an error.

[unional]

This is partially addressed by the new case options, supporting $then/$else/$never and more.