Prevent Android chats from freezing or going blank (#1989)

* fix(app): stabilize streamed chat rendering

Use deterministic Markdown AST keys and preserve unique row identities when an assistant message resumes after a tool. Keep native gesture and plan-card children correctly keyed to avoid unsupported Fabric reconciliation paths.

* fix(app): stabilize retained native panels

Keep retained workspace roots in a stable native order and make active
panels present in the selection commit. Hidden panels now stop expensive
subscriptions and animations without losing mounted state.

* fix(app): harden retained panel activity

packages/app/src/stores/session-store.ts

@@ -1203,7 +1203,10 @@ export const useSessionStore = create<SessionStore>()(
               [serverId]: {
                 ...session,
                 agents: nextAgents,
-                workspaceAgentActivity: buildWorkspaceAgentActivityIndex(nextAgents),
+                workspaceAgentActivity: buildWorkspaceAgentActivityIndex(
+                  nextAgents,
+                  session.workspaceAgentActivity,
+                ),
               },
             },
           };