Function decode_scheme_tree 

pub fn decode_scheme_tree(
    cx: &mut ReadCx<'_>,
    source_id: impl Into<String>,
    input: Input,
) -> Result<LocatedExprTree>

Decodes Scheme surface text into a located Expr tree under codec budgets.

Interns the source, enforces the decode limits, and returns the single top-level form as a LocatedExprTree.