fix(mobile): require a reader for RPC result variants

mobile/src/transport/rpc-operation-compile-fence.ts

@@ -1,6 +1,7 @@
 import type { RpcClient } from './rpc-client'
 import type { RpcMethodName } from './rpc-params-contract'
 import { defineRpcOperation, runRpcOperation, startRpcOperation } from './rpc-operation'
+import { rpcResultVariants } from './rpc-operation-result-reader'
 import {
   workspaceListAtBarrier,
   workspaceListOrNull,
@@ -24,6 +25,9 @@ import type {
 
 declare const client: RpcClient
 
+// @ts-expect-error a variant reader combinator must have at least one reader
+const _fenceEmptyVariantReaders = rpcResultVariants([])
+
 // FENCE: probe-cannot-take-a-reader
 export const fenceProbeWithReader: CapabilityProbeRpcDefinition<'worktree.ps', 'on-settle'> = {
   name: 'fence.probeWithReader',

mobile/src/transport/rpc-operation-result-reader.ts

@@ -39,7 +39,10 @@ export function rpcResultVariant<Variant extends string, Schema extends z.ZodTyp
 
 /** Tries each variant in declared order and takes the first that reads. */
 export function rpcResultVariants<Variant extends string, Value>(
-  readers: readonly NamedRpcResultReader<Variant, Value>[]
+  readers: readonly [
+    NamedRpcResultReader<Variant, Value>,
+    ...NamedRpcResultReader<Variant, Value>[]
+  ]
 ): RpcCompatibleReader<unknown, Variant, Value> {
   return (raw) => {
     const issues: RpcDecodeIssue[] = []