fix(scope): generalize the carrier's proxy-invariant guard; keep the real constructor

Codex follow-up findings on the carrier commit, both verified:

B: the invariant guard only protected the bind path — an overlay key
colliding with a non-configurable, non-writable OWN prop of a (pathological)
base would have the get trap report the overlay value, which the engine
rejects as a proxy invariant violation (TypeError at read time). The pin
check now runs FIRST and covers both invariant-pinned shapes (non-writable
own data prop reported as-is; getterless non-configurable accessor reported
undefined via the delegated read) before overlay and bind alike. Such a base
forgoes scope filtering by construction — correctness of the read beats
filtering for a base no production code ships.

C: `constructor` is looked up for identity, never invoked as a subject
method — binding it broke `carrier.constructor === Subject` for no benefit;
it now returns raw (the same special-case withProps had). Both pinned in
scope.spec: the frozen-own-filter collision yields the base's value without
throwing, and class identity survives the carrier.

packages/core/scope/src/index.ts

@@ -217,17 +217,25 @@ export function scopeTarget<T extends object>(base: T, key: ScopeKey | undefined
   // and sets land on `base` directly.
   return new Proxy(base, {
     get(target, prop) {
+      // Proxy get invariants pin what this trap may report for a
+      // non-configurable OWN property of the base: a non-writable data prop
+      // must be reported AS-IS (neither overlaid nor bound), a getterless
+      // accessor as undefined — checked FIRST so even an overlay key
+      // colliding with a frozen own prop of a (pathological) base yields the
+      // base's value instead of an engine TypeError. Such a base forgoes
+      // scope filtering; no production base freezes these keys.
+      const own = Reflect.getOwnPropertyDescriptor(target, prop)
+      const pinned = own !== undefined && own.configurable === false
+        && own.get === undefined && own.writable !== true
       // hasOwn, not `in`: the overlay literal inherits Object.prototype, so
       // `in` would claim `toString`/`constructor` and shadow the subject's.
-      if (Object.hasOwn(overlay, prop)) return overlay[prop]
+      if (!pinned && Object.hasOwn(overlay, prop)) return overlay[prop]
       const value: unknown = Reflect.get(target, prop, target)
-      if (typeof value !== 'function') return value
-      // Proxy invariant guard: a non-configurable, non-writable OWN data
-      // property must be reported unchanged, so it cannot be bound. Class
-      // methods live on the prototype (no own descriptor) and bind freely;
-      // only a frozen own-function prop keeps the raw (unbound) function.
-      const own = Reflect.getOwnPropertyDescriptor(target, prop)
-      if (own !== undefined && own.configurable === false && own.writable === false) return value
+      if (typeof value !== 'function' || pinned) return value
+      // `constructor` is looked up, never invoked as a subject method — keep
+      // the real one (withProps special-cases it the same way), so
+      // `carrier.constructor` still identifies the subject's class.
+      if (prop === 'constructor') return value
       // `Function.prototype.bind` types as `any`; the value is structurally
       // T[prop] and the trap's contract is untyped (`any`), so unknown is the
       // honest safe return.

packages/core/scope/tests/scope.spec.ts

@@ -222,6 +222,28 @@ describe('scopeTarget dispatch filtering', () => {
     // it from shadowing the subject's own prototype-surface members.
     expect(String(carrier)).toBe('base-str')
   })
+
+  it('honors the get invariant even when an overlay key collides with a frozen own prop of the base', () => {
+    // Pathological but engine-enforced: a base whose own [Context.filter] is
+    // a non-configurable, non-writable data prop pins what any proxy over it
+    // may report for that key. The carrier must yield the base's value (an
+    // overlay there would be a runtime TypeError from the engine, not a
+    // filtering choice). Such a base forgoes scope filtering by construction.
+    const pinnedFilter = (): boolean => true
+    const base = {}
+    Object.defineProperty(base, Context.filter, { value: pinnedFilter, writable: false, configurable: false })
+    const carrier = scopeTarget(base, { name: 'key' })
+    expect((carrier as Record<symbol, unknown>)[Context.filter]).toBe(pinnedFilter)
+  })
+
+  it('keeps the real constructor: class identity survives the carrier', () => {
+    class Subject { work(): string { return 'w' } }
+    const subject = new Subject()
+    const carrier = scopeTarget(subject, subject)
+    // `constructor` is looked up, never invoked as a subject method — binding
+    // it would break `carrier.constructor === Subject` for no benefit.
+    expect(carrier.constructor).toBe(Subject)
+  })
 })
 
 describe('carrier marks', () => {