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-conversation/tests/chat-code-subcalls.spec.tsx

@@ -128,7 +128,7 @@ async function bench(snapshot: ConversationSnapshot) {
   ctx.provide('sessions', sessionsFake)
   const workspaces = {
     list: createSnapshotStore<WorkspaceListState>({
-      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: [], state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     }),
     startSession: vi.fn(),

packages/client/ui-conversation/tests/chat-view.spec.tsx

@@ -94,7 +94,7 @@ function emptySessions() {
 
 function emptyWorkspaces() {
   const store = createSnapshotStore<WorkspaceListState>({
-    items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
+    items: [], archivedSessionIds: [], state: 'idle', phase: 'ready', error: null,
     baselinesReady: true, recentWorkspaceId: undefined,
   })
   return bindSnapshotSelector(store)

packages/client/ui-conversation/tests/gate-branch-tails.spec.tsx

@@ -74,7 +74,7 @@ describe('render branch tails', () => {
     const emptyList = createSnapshotStore<SessionListState>(
       { ids: [], byId: {}, current: undefined, phase: 'ready' })
     const emptyWorkspaces = createSnapshotStore<WorkspaceListState>({
-      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: [], state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })
     const view = render(
@@ -111,7 +111,7 @@ describe('render branch tails', () => {
     const emptyList = createSnapshotStore<SessionListState>(
       { ids: [], byId: {}, current: undefined, phase: 'ready' })
     const emptyWorkspaces = createSnapshotStore<WorkspaceListState>({
-      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: [], state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })
     const view = render(

packages/client/ui-conversation/tests/input-bar.spec.tsx

@@ -96,7 +96,7 @@ function bench(over?: BenchOptions) {
       ids: [], byId: {}, current: undefined, phase: 'ready',
     })),
     useWorkspaces: bindSnapshotSelector(createSnapshotStore({
-      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: [], state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })),
     useProjection: ((key: string, selector?: (v: unknown) => unknown) =>

packages/client/ui-conversation/tests/input-matrix.spec.tsx

@@ -39,7 +39,7 @@ function mountBar(shell: SessionInputShell, over?: { running?: boolean; disabled
       ids: [], byId: {}, current: undefined, phase: 'ready',
     })),
     useWorkspaces: bindSnapshotSelector(createSnapshotStore({
-      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: [], state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })),
     useProjection: (() => undefined),

packages/client/ui-conversation/tests/input-scenarios.spec.tsx

@@ -125,7 +125,7 @@ async function scopedBench(register?: (slash: SlashService) => void) {
       ids: [], byId: {}, current: undefined, phase: 'ready',
     })),
     useWorkspaces: bindSnapshotSelector(createSnapshotStore({
-      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: [], state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })),
     useProjection: (() => undefined),

packages/client/ui-conversation/tests/skeleton.spec.tsx

@@ -62,7 +62,7 @@ function workspace(id = 'w1'): WorkspaceView {
 }
 
 const workspaceState = (items: readonly WorkspaceView[]): WorkspaceListState => ({
-  items, archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
+  items, archivedSessionIds: [], state: 'idle', phase: 'ready', error: null,
   baselinesReady: true, recentWorkspaceId: undefined,
 })
 

packages/client/ui-conversation/tests/terminal-card.spec.tsx

@@ -429,7 +429,7 @@ describe('DetailsPanel Output section', () => {
         phase: 'ready',
       })
     const workspaces = createSnapshotStore<WorkspaceListState>({
-      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: [], state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })
     return render(
@@ -607,7 +607,7 @@ describe('DetailsPanel Output section', () => {
         useSessions={bindSnapshotSelector(createSnapshotStore<SessionListState>(
           { ids: [], byId: {}, current: undefined, phase: 'ready' }))}
         useWorkspaces={bindSnapshotSelector(createSnapshotStore<WorkspaceListState>({
-          items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
+          items: [], archivedSessionIds: [], state: 'idle', phase: 'ready', error: null,
           baselinesReady: true, recentWorkspaceId: undefined,
         }))}
         useInput={(() => { throw new Error('unused') })}