An arbitrary-precision decimal serialized in plain (non-scientific) notation. The default `:decimal` scalar delegates to `Decimal.to_string/1`, which uses `:scientific` notation in `Decimal` v2 (e.g. `1E-9`). Microdose values must reach the UI as a human-readable string like `0.000000001`, so this scalar forces `:normal` notation on serialize. Parsing is identical to `:decimal`.