Source plugin for polytropos.

Desugars ergonomic syntax for row-polymorphic records, open variants, and declaration forms into standard Polytropos runtime combinators before renaming and typechecking:

1. Record Construction: REC { x = 1, y = True } (bare) and REC_F { x = Just 1 } (HKD) with field punning support.
2. Variant Injection: VAR { Ok = 200 } or VARIANT { Err = "fail" }.
3. Record Updates: REC_UPD r { x = 42 } into setField chains.
4. Pattern Matching: REC { x, y } into record view patterns, and VAR { Tag = pat } into variant projection view patterns.
5. Open Declarations: data Person = Person { name :: String, age :: Int } deriving OpenRecord and data Status = Active | Inactive String deriving OpenVariant.