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/ui-layout/tests/app-frame.spec.tsx

@@ -77,7 +77,7 @@ function mountFrame() {
     return sel(sessionState)
   }) as never
   const workspaceState: WorkspaceListState = {
-    items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
+    items: [], archivedSessionIds: [], state: 'idle', phase: 'ready', error: null,
     baselinesReady: baselinesReady.current, recentWorkspaceId: undefined,
   }
   const element = () => (