feat(client-runtime): project the archive set and the archiveSession action

WorkspaceListState gains archivedSessionIds (ReadonlySet, replaced only
on membership change), installed as full snapshots from the list
baseline, the unary echo, and the changed frame. Archiving the current
session clears the selection into the New Session view state. Test
doubles (test-runtime, fake APIs, fixture client) follow the widened
IWorkspaces/IApiClient faces.

packages/client/runtime/tests/workspaces-service.spec.ts

@@ -285,6 +285,52 @@ describe('WorkspacesService', () => {
     }))
     await expect(workspaces.delete(wid('ghost'))).rejects.toThrow(/workspace-not-found: gone/)
   })
+
+  it('archives a session, projects the set from the response, list, and frame, and clears only the current one', async () => {
+    const ctx = new Context()
+    const api = new FakeApiClient()
+    const sessions = new SessionsService(ctx, api)
+    const workspaces = new WorkspacesService(ctx, api, sessions)
+    api.onList = () => Promise.resolve(ok({
+      items: [
+        { sessionId: sid('s-open'), updatedAt: 2, running: false, blank: false },
+        { sessionId: sid('s-idle'), updatedAt: 1, running: false, blank: false },
+      ],
+    }) as never)
+    await sessions.refresh()
+    sessions.open(sid('s-open'))
+
+    // Archiving a non-current session installs the unary echo and keeps the selection.
+    await expect(workspaces.archiveSession(sid('s-idle'))).resolves.toBeUndefined()
+    expect(api.callsOf('workspace.archiveSession')).toEqual([{ sessionId: 's-idle' }])
+    expect([...workspaces.list.getSnapshot().archivedSessionIds]).toEqual(['s-idle'])
+    expect(sessions.list.getSnapshot().current).toBe('s-open')
+
+    // Archiving the current session clears it into the New Session view state.
+    api.onWorkspaceArchiveSession = () => Promise.resolve(ok({ archivedSessionIds: [sid('s-idle'), sid('s-open')] }))
+    await workspaces.archiveSession(sid('s-open'))
+    expect([...workspaces.list.getSnapshot().archivedSessionIds]).toEqual(['s-idle', 's-open'])
+    expect(sessions.list.getSnapshot().current).toBeUndefined()
+
+    // A Host failure leaves the set and the selection untouched.
+    api.onWorkspaceArchiveSession = () => Promise.resolve(err({
+      code: 'session-not-found', message: 'no session ghost', details: { sessionId: sid('ghost') },
+    }))
+    await expect(workspaces.archiveSession(sid('ghost'))).rejects.toThrow(/session-not-found/)
+    expect([...workspaces.list.getSnapshot().archivedSessionIds]).toEqual(['s-idle', 's-open'])
+
+    // The changed frame and the list baseline both re-install the full set.
+    workspaces.handleHostEnvelope({
+      rpcId: 'frame' as never,
+      payload: { type: 'host/archived-sessions-changed', archivedSessionIds: [sid('s-idle')] },
+    } as never)
+    // Frame installs ride the notifier's microtask batch before projecting.
+    await new Promise(resolve => setTimeout(resolve, 0))
+    expect([...workspaces.list.getSnapshot().archivedSessionIds]).toEqual(['s-idle'])
+    api.onWorkspaceList = () => Promise.resolve(ok({ items: [], archivedSessionIds: [sid('s-open')] }) as never)
+    await workspaces.refresh()
+    expect([...workspaces.list.getSnapshot().archivedSessionIds]).toEqual(['s-open'])
+  })
 })
 
 describe('startInitialSelection', () => {