fix(ocap-kernel): use KernelCapDataStruct in RPC specs to eliminate unsafe casts

Replace CapDataStruct (validates string[] slots) with
KernelCapDataStruct (validates KRef[] slots) in queueMessage and
launchSubcluster RPC specs. This makes the runtime validation match the
declared CapData<KRef> type, eliminating two `as unknown as` casts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rekmarks

packages/ocap-kernel/src/rpc/kernel-control/launch-subcluster.ts

@@ -1,13 +1,12 @@
 import type { CapData } from '@endo/marshal';
 import type { MethodSpec, Handler } from '@metamask/kernel-rpc-methods';
 import { object, nullable, type as structType } from '@metamask/superstruct';
-import type { Struct } from '@metamask/superstruct';
 
 import type { Kernel } from '../../Kernel.ts';
 import type { ClusterConfig, KRef, SubclusterId } from '../../types.ts';
 import {
   ClusterConfigStruct,
-  CapDataStruct,
+  KernelCapDataStruct,
   SubclusterIdStruct,
   KRefStruct,
 } from '../../types.ts';
@@ -25,7 +24,7 @@ type LaunchSubclusterRpcResult = {
 const LaunchSubclusterRpcResultStruct = structType({
   subclusterId: SubclusterIdStruct,
   rootKref: KRefStruct,
-  bootstrapResult: nullable(CapDataStruct),
+  bootstrapResult: nullable(KernelCapDataStruct),
 });
 
 export const launchSubclusterSpec: MethodSpec<
@@ -35,10 +34,7 @@ export const launchSubclusterSpec: MethodSpec<
 > = {
   method: 'launchSubcluster',
   params: object({ config: ClusterConfigStruct }),
-  result: LaunchSubclusterRpcResultStruct as unknown as Struct<
-    LaunchSubclusterRpcResult,
-    unknown
-  >,
+  result: LaunchSubclusterRpcResultStruct,
 };
 
 export type LaunchSubclusterHooks = {

packages/ocap-kernel/src/rpc/kernel-control/queue-message.ts

@@ -6,7 +6,7 @@ import type { Json } from '@metamask/utils';
 
 import type { Kernel } from '../../Kernel.ts';
 import type { KRef } from '../../types.ts';
-import { CapDataStruct, KRefStruct } from '../../types.ts';
+import { KernelCapDataStruct, KRefStruct } from '../../types.ts';
 
 /**
  * Enqueue a message to a vat via the kernel's crank queue.
@@ -18,12 +18,8 @@ export const queueMessageSpec: MethodSpec<
 > = {
   method: 'queueMessage',
   params: tuple([KRefStruct, string(), array(UnsafeJsonStruct)]),
-  result: CapDataStruct,
-} as unknown as MethodSpec<
-  'queueMessage',
-  [KRef, string, Json[]],
-  CapData<KRef>
->;
+  result: KernelCapDataStruct,
+};
 
 export type QueueMessageHooks = {
   kernel: Pick<Kernel, 'queueMessage'>;