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-workspace/tests/workspace-picker.spec.tsx

@@ -32,7 +32,7 @@ const sessions: SessionListState = {
   ids: [], byId: {}, current: undefined, phase: 'ready',
 }
 const workspaceState = (items: readonly WorkspaceView[]): WorkspaceListState => ({
-  items, archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null, baselinesReady: true,
+  items, archivedSessionIds: [], state: 'idle', phase: 'ready', error: null, baselinesReady: true,
   recentWorkspaceId: items[0]?.workspaceId,
 })
 function anchor(): { current: HTMLElement } {