iter rpe.1.tidy: subst.rs preserves Type::Fn modes through rebuild

Closes audit-rpe.1's [medium] drift: qualify_local_types_codegen
and apply_subst_to_type at crates/ailang-codegen/src/subst.rs:188 +
:217 both carried the field-spread Type::Fn rebuild shape that
strips param_modes and ret_mode to vec![] / ParamMode::Implicit.
Symmetric to the feb9413 substitute_rigids fix flagged in that
commit's message as a bug class.

Fix: bind param_modes and ret_mode in the destructure, thread
them through unchanged. cargo test --workspace 564/0/3; clippy + doc
zero warnings.

Audit-rpe.1's [nit] dead Emitter.strings field deferred as
known-debt — it cycles over an empty map harmlessly.

Three other field-spread Type::Fn rebuild sites in
ailang-check/src/lib.rs and lift.rs follow the same pattern but
are not currently flagged by failing tests; left in place pending
correctness-reachability verification per the journal Known debt
section.

crates/ailang-codegen/src/subst.rs

@@ -185,15 +185,15 @@ pub(crate) fn qualify_local_types_codegen(
                     .collect(),
             }
         }
-        Type::Fn { params, ret, effects, .. } => Type::Fn {
+        Type::Fn { params, ret, effects, param_modes, ret_mode } => Type::Fn {
             params: params
                 .iter()
                 .map(|p| qualify_local_types_codegen(p, owner_module, owner_local_types))
                 .collect(),
             ret: Box::new(qualify_local_types_codegen(ret, owner_module, owner_local_types)),
             effects: effects.clone(),
-            param_modes: vec![],
-            ret_mode: ParamMode::Implicit,
+            param_modes: param_modes.clone(),
+            ret_mode: *ret_mode,
         },
         Type::Forall { vars, constraints, body } => Type::Forall {
             vars: vars.clone(),
@@ -214,12 +214,12 @@ pub(crate) fn apply_subst_to_type(t: &Type, subst: &BTreeMap<String, Type>) -> T
             name: name.clone(),
             args: args.iter().map(|a| apply_subst_to_type(a, subst)).collect(),
         },
-        Type::Fn { params, ret, effects, .. } => Type::Fn {
+        Type::Fn { params, ret, effects, param_modes, ret_mode } => Type::Fn {
             params: params.iter().map(|p| apply_subst_to_type(p, subst)).collect(),
             ret: Box::new(apply_subst_to_type(ret, subst)),
             effects: effects.clone(),
-            param_modes: vec![],
-            ret_mode: ParamMode::Implicit,
+            param_modes: param_modes.clone(),
+            ret_mode: *ret_mode,
         },
         Type::Forall { vars, constraints, body } => {
             // Inner forall shadows: don't substitute re-bound names.