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__/debouncedStorage.test.ts

@@ -0,0 +1,72 @@
+import { createDebouncedStorage } from '../store/debouncedStorage';
+
+const makeBase = () => ({
+    getItem: jest.fn(async (_name: string): Promise<string | null> => 'from-disk'),
+    setItem: jest.fn(async (_name: string, _value: string) => {}),
+    removeItem: jest.fn(async (_name: string) => {}),
+});
+
+beforeEach(() => {
+    jest.useFakeTimers();
+});
+afterEach(() => {
+    jest.useRealTimers();
+});
+
+describe('createDebouncedStorage', () => {
+    it('collapses a burst of writes into one trailing write with the latest value', () => {
+        const base = makeBase();
+        const storage = createDebouncedStorage(base, 400);
+
+        storage.setItem('k', '1');
+        jest.advanceTimersByTime(300);
+        storage.setItem('k', '2');
+        jest.advanceTimersByTime(300);
+        storage.setItem('k', '3');
+        expect(base.setItem).not.toHaveBeenCalled();
+
+        jest.advanceTimersByTime(400);
+        expect(base.setItem).toHaveBeenCalledTimes(1);
+        expect(base.setItem).toHaveBeenCalledWith('k', '3');
+    });
+
+    it('reads a pending value before it reaches the disk, and falls back to the base storage', async () => {
+        const base = makeBase();
+        const storage = createDebouncedStorage(base);
+
+        await expect(storage.getItem('k')).resolves.toBe('from-disk');
+        storage.setItem('k', 'fresh');
+        await expect(storage.getItem('k')).resolves.toBe('fresh');
+    });
+
+    it('flush writes immediately and cancels the timer', async () => {
+        const base = makeBase();
+        const storage = createDebouncedStorage(base);
+
+        storage.setItem('a', '1');
+        storage.setItem('b', '2');
+        await storage.flush();
+        expect(base.setItem.mock.calls).toEqual([['a', '1'], ['b', '2']]);
+
+        jest.advanceTimersByTime(1000);
+        expect(base.setItem).toHaveBeenCalledTimes(2);
+    });
+
+    it('flush with nothing pending does not write', async () => {
+        const base = makeBase();
+        await createDebouncedStorage(base).flush();
+        expect(base.setItem).not.toHaveBeenCalled();
+    });
+
+    it('removeItem drops a pending write and removes from the base storage', async () => {
+        const base = makeBase();
+        const storage = createDebouncedStorage(base);
+
+        storage.setItem('k', '1');
+        await storage.removeItem('k');
+        jest.advanceTimersByTime(1000);
+
+        expect(base.setItem).not.toHaveBeenCalled();
+        expect(base.removeItem).toHaveBeenCalledWith('k');
+    });
+});