refactor(client): archivedSessionIds public face becomes a plain array

Public snapshot state stays in the store engine's plain-data vocabulary
(immer drafts reject Sets without the MapSet plugin, which stays off):
manager/service/contract carry readonly SessionId[] in Host order, and
the tree derivations build their own transient Set — the
expandedProjects pattern. Membership-unchanged installs still keep the
array reference for Object.is short-circuits.

packages/client/test-runtime/src/fixtures.ts

@@ -73,7 +73,7 @@ export function conversationSnapshot(sessionId: SessionId): ConversationSnapshot
 export function workspaceListState(): WorkspaceListState {
   return {
     items: [],
-    archivedSessionIds: new Set(),
+    archivedSessionIds: [],
     state: 'idle',
     phase: 'ready',
     error: null,

packages/client/test-runtime/src/workspaces.ts

@@ -199,11 +199,8 @@ export class TestWorkspaces implements IWorkspaces {
       await (stub(sessionId) as Promise<void>)
       return
     }
-    // Built outside the draft: reading a Set through an immer draft needs
-    // the MapSet plugin, while assigning a fresh Set does not.
-    const next = new Set([...this.list.getSnapshot().archivedSessionIds, sessionId])
     await this.update((draft) => {
-      draft.archivedSessionIds = next
+      draft.archivedSessionIds = [...draft.archivedSessionIds, sessionId]
     })
   }
 }