Fix: Apply substitutions to upvalue types

When an identifier is used as an upvalue within a nested scope (e.g., a
`while` loop), the type checker needs to apply the global substitution
map to resolve `TypeVar`s that might have been determined in outer
scopes. This ensures that the correct type is used even if
`ctx.set_type` couldn't propagate the change directly through the
upvalue address.

This fix also includes a regression test to specifically cover this
scenario with series and closures.

src/ast/compiler/type_checker.rs

@@ -906,7 +906,10 @@ impl TypeChecker {
 
             NodeKind::Identifier { symbol, binding } => {
                 if let IdentifierBinding::Reference(addr) = binding {
-                    let ty = ctx.get_type(*addr);
+                    // Apply the current HM substitution so that TypeVars resolved in
+                    // nested scopes (e.g. inside a `while` body) are visible here even
+                    // when ctx.set_type could not propagate back through an upvalue address.
+                    let ty = Self::apply_subst(ctx.get_type(*addr), &self.subst.borrow());
                     (
                         NodeKind::Identifier {
                             symbol: symbol.clone(),
@@ -915,7 +918,7 @@ impl TypeChecker {
                         ty,
                     )
                 } else if let IdentifierBinding::Declaration { addr, kind: decl_kind } = binding {
-                    let ty = ctx.get_type(*addr);
+                    let ty = Self::apply_subst(ctx.get_type(*addr), &self.subst.borrow());
                     (
                         NodeKind::Identifier {
                             symbol: symbol.clone(),