docs: add doc comment for FnParamDef.unresolved_ty reserved for future use

Author: furkanmamuk

crates/vexil-lang/src/ir/mod.rs

@@ -375,6 +375,8 @@ pub struct TraitFnDef {
 pub struct FnParamDef {
     pub name: SmolStr,
     pub ty: ResolvedType,
+    /// Original AST type expression, preserved for trait function param validation.
+    /// Currently unused but reserved for future trait function signature conformance checking.
     pub unresolved_ty: crate::ast::TypeExpr,
 }