refactor: une seule fonction decide ce qu-une mutation peut rendre

Trois copies d-un raisonnement sur une valeur de retour, c-est trois occasions
de se tromper et c-est ce qui a produit le defaut. `handBack` porte la regle
une fois : la collection devient la vue, un scalaire passe, le reste refuse.

packages/polyfill/src/emulated-verifier/read-filter.ts

@@ -243,6 +243,19 @@ export function makeReadFilteredView<S extends object>(set: S, caps: CapRegistry
     return self.view;
   };
 
+  /**
+   * What a MUTATION may hand back - the same question this file asks of any other member,
+   * asked of a RESULT rather than of a property. Written once and called from each of
+   * them, because the defect it answers was three copies of a reasoned-about return value:
+   * the collection becomes the view, a scalar passes, and anything else refuses rather
+   * than being guessed at.
+   */
+  const handBack = (out: unknown, target: object, prop: string | symbol): unknown => {
+    if (out === target) return viewOf(prop);
+    if (cannotReachAnItem(out)) return out;
+    throw refusalOf(prop, "the collection view");
+  };
+
   /** What the view answers for one member. The `get` trap and the descriptor trap BOTH
    *  call this, so neither can answer what the other refuses. */
   const read = (target: object, prop: string | symbol): unknown => {
@@ -334,12 +347,9 @@ export function makeReadFilteredView<S extends object>(set: S, caps: CapRegistry
     if (prop === "add") {
       const fn = Reflect.get(target, prop, target);
       if (typeof fn !== "function") return fn;
-      return (...args: unknown[]): unknown => {
-        const out = (fn as (...a: unknown[]) => unknown).apply(target, args);
-        if (out === target) return viewOf(prop); // chaining lands on the view
-        if (cannotReachAnItem(out)) return out;
-        throw refusalOf(prop, "the collection view");
-      };
+      // `add` chains, so its result is the collection and must become the view.
+      return (...args: unknown[]): unknown =>
+        handBack((fn as (...a: unknown[]) => unknown).apply(target, args), target, prop);
     }
     if (prop === "delete") {
       const fn = Reflect.get(target, prop, target);
@@ -347,7 +357,7 @@ export function makeReadFilteredView<S extends object>(set: S, caps: CapRegistry
       // The same question `has` already refuses to answer, with a worse consequence. The
       // two disagreed: `has(item)` said no while `delete(item)` removed it.
       return (item: unknown): unknown =>
-        keep(item) ? (fn as (i: unknown) => unknown).call(target, item) : false;
+        keep(item) ? handBack((fn as (i: unknown) => unknown).call(target, item), target, prop) : false;
     }
     if (prop === "clear") {
       const del = Reflect.get(target, "delete", target);