fix(scope): harden final ownership boundaries

docs/core-data-structures/scope.md

@@ -0,0 +1,31 @@
+# Scoped Registration
+
+The [scope package](../../packages/core/scope) supplies the identity and carrier vocabulary that makes one registration context mean both per-agent visibility and shared lifetime ownership. It is a library primitive rather than a Cordis service; the [agent-scope RFC](../rfc/implemented/architecture/2026-07-08-agent-scope-contexts.md) owns the design rationale, while the package [README](../../packages/core/scope/README.md) owns the callable API and filtering semantics.
+
+Source: [`packages/core/scope/src/index.ts`](../../packages/core/scope/src/index.ts).
+
+## Identity and dispatch carrier
+
+`ScopeKey` is an opaque object identity. The shipped loop uses the live `Agent` object as its own key, but the primitive never inspects the object.
+
+```ts type-equiv
+type ScopeKey = object
+```
+
+`Scoped<T>` is the compile-time brand on the proxy returned by `scopeTarget(base, key)`. Scope-filtered event declarations require this carrier as their `this` type, preventing an ordinary subject object from type-checking as the dispatch carrier.
+
+```ts type-equiv
+type Scoped<T> = T & { readonly [ScopedBrand]: 'dsh.scope.carrier' }
+```
+
+## Owned registration context
+
+`Scope` pairs the tagged registration context with two teardown surfaces. `rawDispose` preserves the exact Cordis disposer identity needed by an ordered composite effect; `dispose()` is the public shared quiescence boundary for direct and racing callers.
+
+```ts type-equiv
+interface Scope {
+  ctx: Context
+  rawDispose: () => Promise<void> | void
+  dispose(): Promise<void>
+}
+```