fix: le prototype et l-extensibilite sont des ecritures, elles aussi

Trouve en balayant le correctif de ce round plutot qu-en raisonnant dessus.
`Object.setPrototypeOf(vue, ...)` remplacait le prototype de la VRAIE
collection - apres quoi l-iterer leve pour toutes les identites, c-est le
probleme de `clear` dans une autre tonalite - et `Object.preventExtensions`,
par quoi `Object.freeze` commence, la rendait non extensible : une ecriture, et
de surcroit la projection d-index de `ownKeys` n-est licite que tant que la
cible est extensible.

packages/polyfill/test/read-filter.test.ts

@@ -366,6 +366,15 @@ test("a write to the view itself is refused, whatever the collection underneath
     view.length = 0;
   }).toThrow(/not filtered/i);
   expect(() => Object.defineProperty(view, "0", { value: { id: "SPY" } })).toThrow(/not filtered/i);
+  // The two that reach the target without naming a property. Replacing the prototype
+  // breaks iteration of the real collection for EVERY identity; `preventExtensions` -
+  // which `Object.freeze` starts with - writes to it and unlawfully strands the index
+  // projection. Both were found by sweeping this view's own repair.
+  expect(() => Object.setPrototypeOf(view, { stolen: () => items })).toThrow(/not filtered/i);
+  expect(() => Object.preventExtensions(view)).toThrow(/not filtered/i);
+  expect(() => Object.freeze(view)).toThrow(/not filtered/i);
   // The plain array underneath accepted nothing, so the view is what stood in the way.
   expect(items.map((i) => i.id)).toEqual(["a", "p"]);
+  expect(Object.getPrototypeOf(items)).toBe(Array.prototype);
+  expect(Object.isExtensible(items)).toBe(true);
 });