perf(chat): debounce persisted writes and re-render only the streaming message

- the Zustand persist middleware serialized every saved session and wrote to AsyncStorage on each streamed token (appendToMessage -> set); writes are now debounced (400 ms) and flushed when the app leaves the foreground
- every token re-rendered every MessageBubble, re-parsing the Markdown of the whole conversation; rows are memoized so only the changed message renders (covered by MessageList.test.tsx)

__tests__/MessageList.test.tsx

@@ -0,0 +1,78 @@
+import { render, screen } from '@testing-library/react-native';
+import React from 'react';
+import { MessageList } from '../components/chat/MessageList';
+import { ChatMessage } from '../store/useAppStore';
+
+jest.mock('react-native-reanimated', () => {
+    const { View } = require('react-native');
+    const chain: any = new Proxy(() => chain, { get: () => chain });
+    return { __esModule: true, default: { View }, FadeInDown: chain };
+});
+
+const mockRenders: Record<string, number> = {};
+jest.mock('../components/chat/MessageBubble', () => {
+    const { Text } = require('react-native');
+    return {
+        MessageBubble: ({ message }: any) => {
+            mockRenders[message.id] = (mockRenders[message.id] ?? 0) + 1;
+            return <Text>{`${message.id}:${message.content}`}</Text>;
+        },
+    };
+});
+jest.mock('../components/chat/TypingIndicator', () => {
+    const { Text } = require('react-native');
+    return { TypingIndicator: () => <Text>typing-indicator</Text> };
+});
+
+const msg = (id: string, role: ChatMessage['role'], content: string): ChatMessage => ({
+    id,
+    role,
+    content,
+    timestamp: 0,
+});
+
+const handlers = { onEdit: jest.fn(), onDelete: jest.fn(), onRegenerate: jest.fn() };
+
+beforeEach(() => {
+    Object.keys(mockRenders).forEach((k) => delete mockRenders[k]);
+});
+
+describe('MessageList', () => {
+    it('renders every message', () => {
+        render(
+            <MessageList
+                messages={[msg('u1', 'user', 'q'), msg('a1', 'assistant', 'a')]}
+                isStreaming={false}
+                {...handlers}
+            />,
+        );
+        expect(screen.getByText('u1:q')).toBeTruthy();
+        expect(screen.getByText('a1:a')).toBeTruthy();
+    });
+
+    it('re-renders only the message that changed while an answer streams in', () => {
+        const user = msg('u1', 'user', 'question');
+        const earlier = msg('a0', 'assistant', 'earlier answer');
+        const streaming = msg('a1', 'assistant', 'He');
+
+        const { rerender } = render(
+            <MessageList messages={[user, earlier, streaming]} isStreaming {...handlers} />,
+        );
+        expect(mockRenders).toEqual({ u1: 1, a0: 1, a1: 1 });
+
+        rerender(
+            <MessageList messages={[user, earlier, { ...streaming, content: 'Hello' }]} isStreaming {...handlers} />,
+        );
+        expect(screen.getByText('a1:Hello')).toBeTruthy();
+        expect(mockRenders.u1).toBe(1);
+        expect(mockRenders.a0).toBe(1);
+        expect(mockRenders.a1).toBe(2);
+    });
+
+    it('shows the typing indicator while the last assistant message is still empty', () => {
+        render(
+            <MessageList messages={[msg('u1', 'user', 'q'), msg('a1', 'assistant', '')]} isStreaming {...handlers} />,
+        );
+        expect(screen.getByText('typing-indicator')).toBeTruthy();
+    });
+});