refactor(prose): fold the five copies of effects-clause rendering into one helper

The " with <e1>, <e2>" effects clause was rendered by five verbatim
blocks across the fn / class-method / instance-method / type writers.
Hoisted into write_effects_clause(out, effects); all five sites now
call it. Output is byte-identical — the snapshot/round-trip tests
confirm no drift.

The two forall<...> blocks were left alone: they diverge in the
trailing token (one ends `>`, the other `> ` before a where-clause),
so a shared helper would be awkward rather than cleaner.

crates/ailang-prose/src/lib.rs

@@ -303,18 +303,7 @@ fn write_fn_def(out: &mut String, fd: &FnDef, level: usize, owning_module: &str)
         }
         out.push_str(") -> ");
         write_mode_type(out, ret, *ret_mode, owning_module);
-        if !effects.is_empty() {
+        write_effects_clause(out, effects);
-            out.push_str(" with ");
-            // Stable order: as written in the AST. Effects are
-            // semantically a set, but re-sorting would erase
-            // author-asserted ordering; preserve the AST order.
-            for (i, e) in effects.iter().enumerate() {
-                if i > 0 {
-                    out.push_str(", ");
-                }
-                out.push_str(e);
-            }
-        }
     } else {
         // Defensive fallback: a non-Fn fn-type. Render the bare type so
         // nothing crashes; this path is unreachable for typechecked
@@ -389,15 +378,7 @@ fn write_class_method(out: &mut String, m: &ClassMethod, level: usize, owning_mo
         }
         out.push_str(") -> ");
         write_mode_type(out, ret, *ret_mode, owning_module);
-        if !effects.is_empty() {
+        write_effects_clause(out, effects);
-            out.push_str(" with ");
-            for (i, e) in effects.iter().enumerate() {
-                if i > 0 {
-                    out.push_str(", ");
-                }
-                out.push_str(e);
-            }
-        }
     } else {
         // Defensive fallback: a non-Fn class-method type. Render the
         // bare type so nothing crashes; this path is unreachable for
@@ -451,6 +432,25 @@ fn write_instance_method(out: &mut String, m: &InstanceMethod, level: usize, own
 
 // ---- types ----------------------------------------------------------------
 
+/// Render the trailing effects clause of a fn / method / lambda signature.
+///
+/// Emits nothing for an empty effect set; otherwise ` with ` followed by
+/// the comma-separated effect names in AST order. Effects are
+/// semantically a set, but re-sorting would erase author-asserted
+/// ordering, so the AST order is preserved verbatim.
+fn write_effects_clause(out: &mut String, effects: &[String]) {
+    if effects.is_empty() {
+        return;
+    }
+    out.push_str(" with ");
+    for (i, e) in effects.iter().enumerate() {
+        if i > 0 {
+            out.push_str(", ");
+        }
+        out.push_str(e);
+    }
+}
+
 fn write_mode_type(out: &mut String, t: &Type, mode: ParamMode, owning_module: &str) {
     match mode {
         ParamMode::Own => {
@@ -511,15 +511,7 @@ fn write_type(out: &mut String, t: &Type, owning_module: &str) {
             }
             out.push_str(") -> ");
             write_mode_type(out, ret, *ret_mode, owning_module);
-            if !effects.is_empty() {
+            write_effects_clause(out, effects);
-                out.push_str(" with ");
-                for (i, e) in effects.iter().enumerate() {
-                    if i > 0 {
-                        out.push_str(", ");
-                    }
-                    out.push_str(e);
-                }
-            }
         }
         Type::Forall { vars, constraints: _, body } => {
             out.push_str("forall<");
@@ -742,15 +734,7 @@ fn write_term_prec(out: &mut String, t: &Term, level: usize, parent_prec: u8, ow
                 }
                 out.push_str(") -> ");
                 write_mode_type(out, ret, *ret_mode, owning_module);
-                if !effects.is_empty() {
+                write_effects_clause(out, effects);
-                    out.push_str(" with ");
-                    for (i, e) in effects.iter().enumerate() {
-                        if i > 0 {
-                            out.push_str(", ");
-                        }
-                        out.push_str(e);
-                    }
-                }
             } else {
                 out.push_str(") -> ");
                 write_type(out, ty, owning_module);
@@ -853,15 +837,7 @@ fn write_term_prec(out: &mut String, t: &Term, level: usize, parent_prec: u8, ow
             }
             out.push_str("| -> ");
             write_type(out, ret_ty, owning_module);
-            if !effects.is_empty() {
+            write_effects_clause(out, effects);
-                out.push_str(" with ");
-                for (i, e) in effects.iter().enumerate() {
-                    if i > 0 {
-                        out.push_str(", ");
-                    }
-                    out.push_str(e);
-                }
-            }
             out.push_str(" {\n");
             indent(out, level + 1);
             write_term(out, body, level + 1, owning_module);