fix(scope): harden final ownership boundaries

packages/core/scope/README.md

@@ -9,7 +9,7 @@ Scoped-context registration primitive. `createScope(ctx, key)` mints a Cordis co
 - `Scope.rawDispose` The EXACT Cordis disposer for the backing fiber — a composite (generator) effect yields THIS function to nest the scope's teardown at that yield position (Cordis dedupes nested effects by function identity; yielding a wrapper leaves the scope disposing as a concurrent sibling).
 - `Scope.dispose(): Promise<void>` Idempotent, shared quiescence boundary for every registration made through the scope. Racing/repeat calls await the same teardown, including when `rawDispose` invoked the underlying single-shot Cordis disposer first.
 - `scopeOf(ctx: Context): ScopeKey | undefined` The tag a context (or any context derived from it) carries; `undefined` = context-global.
-- `scopeTarget(base: T, key?: ScopeKey): Scoped<T>` Build the dispatch `thisArg` for a scope-filtered event: composes `base`'s own `Context.filter` with the scope predicate (untagged listener ⇒ admitted; tagged ⇒ admitted iff tag === key; `key === undefined` ⇒ untagged only). Listener `this` stays `base`-shaped. `{ global: true }` listeners bypass filtering (Cordis semantics).
+- `scopeTarget(base: T, key: ScopeKey | undefined): Scoped<T>` Build the dispatch `thisArg` for a scope-filtered event: capture and compose `base`'s own `Context.filter` with the scope predicate (untagged listener ⇒ admitted; tagged ⇒ admitted iff tag === key; `key === undefined` ⇒ untagged only). The carrier uses a dedicated surrogate proxy target whose immutable filter slot cannot be replaced by a base property pinned before, during, or after construction; ordinary property access, writes, own-key visibility, methods, invocation, and construction delegate to `base`, and callable carriers match the base's constructable/non-constructable shape. For non-overlay base-owned properties, descriptor queries preserve values and flags except that configurable is normalized to `true`, as required to report those properties through an extensible surrogate. Listener `this` stays `base`-shaped. `{ global: true }` listeners bypass filtering (Cordis semantics).
 - `Scoped<T>` The compile-time carrier brand: scope-filtered events demand it as their `this` type, so dispatching with a bare subject is a compile error.
 - `isScopeCarrier(value)` / `carrierKeyOf(value)` Runtime carrier marks, used by the dev invariants to assert every scope-filtered dispatch carries a carrier keyed to the subject its arguments name.
 - `scopeHost(ctx, services)` Test/tooling host that snapshots the requested service list before activation, fails loud with stable missing-service diagnostics, and whose shared `dispose()` waits for both the host fiber and every minted scope, including a child already tearing down through `rawDispose`.