feat(exif): write the Xiaomi watermark blob (0x889e) so HyperOS Gallery can frame our captures

HyperOS Gallery reads EXIF 0x889e (a JSON blob) to build its watermark
frame; without it a photo fails with "cannot recognize the parameters".
CameraX/HAL never supplies that vendor tag, so a capture from this app
lacked it while a stock-camera photo carried it.

Read back the same props the stock camera derives the blob from
(ro.product.device / ro.product.marketname / Build.MANUFACTURER) via the
native module, synthesize the blob in exifWrite, and write it only when
the source has none and the device is Xiaomi/Redmi/POCO - a source blob
is never overwritten.

src/utils/exportEngine.ts

@@ -10,7 +10,7 @@ import { CINEMA_SKSL, getCinemaUniforms, cinemaIsActive } from './cinemaShader';
 import { drawFrameOnCanvas, polaroidLayout, POLAROID_CARD, POLAROID_WIN_W, wallframeLayout, WALLFRAME_W, WALLFRAME_H, WALLFRAME_LAND_W, WALLFRAME_LAND_H } from './frameUtils';
 import { patchJpegDpi } from './jpegDpi';
 import { writeJpegExif } from './exifWrite';
-import { nextPhotoPath, readPhotoBytes } from './photoMeta';
+import { nextPhotoPath, readPhotoBytes, getDeviceIdentity } from './photoMeta';
 import { applyPhotoRotation } from './skiaImage';
 
 export interface ExportResult {
@@ -721,6 +721,7 @@ export async function processAndExportPhoto(
         width: resultImage.width(),
         height: resultImage.height(),
         dpi: options?.dpi ?? null,
+        device: getDeviceIdentity(),
       },
       await readPhotoBytes(sourceUri)
     );