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-browser.spec.tsx

@@ -35,7 +35,7 @@ const workspace = (id: string, sessionIds: string[], title = id): WorkspaceView
   workspaceId: wid(id), path: `/projects/${id}`, title,
   sessionIds: sessionIds.map(sid), createdAt: '2026-01-01T00:00:00.000Z', updatedAt: '2026-01-01T00:00:00.000Z',
 })
-const workspaceState = (items: readonly WorkspaceView[], archivedSessionIds: ReadonlySet<SessionId> = new Set()): WorkspaceListState => ({
+const workspaceState = (items: readonly WorkspaceView[], archivedSessionIds: readonly SessionId[] = []): WorkspaceListState => ({
   items, archivedSessionIds, state: 'idle', phase: 'ready', error: null, baselinesReady: true,
   recentWorkspaceId: items[0]?.workspaceId,
 })
@@ -149,7 +149,7 @@ describe('WorkspaceBrowser', () => {
     expect(archiveSession).toHaveBeenCalledWith(sid('gone-s'))
 
     // The archive-set echo hides the row in grouped mode (count included) and flat mode.
-    rerender(b, { useWorkspaces: hook(workspaceState([workspace('alpha', ['kept-s', 'gone-s'])], new Set([sid('gone-s')]))) })
+    rerender(b, { useWorkspaces: hook(workspaceState([workspace('alpha', ['kept-s', 'gone-s'])], [sid('gone-s')])) })
     expect(screen.queryByText('gone-s')).toBeNull()
     expect(screen.getByText('1 个会话')).toBeTruthy()
     fireEvent.click(screen.getByRole('button', { name: '分组方式' }))