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/exifWrite.ts

@@ -25,6 +25,17 @@ export interface ExifStamp {
   width?: number | null;
   height?: number | null;
   dpi?: number | null;
+  // Device identity for the Gallery watermark blob (ref. watermarkBlob). Only
+  // used when the source photo carries no blob of its own.
+  device?: DeviceIdentity | null;
+}
+
+/** Raw device identity for the HyperOS Gallery watermark tag (0x889e). */
+export interface DeviceIdentity {
+  device: string; // ro.product.device codename, e.g. "aurora"
+  marketName: string; // ro.product.marketname, e.g. "Xiaomi 14 Ultra"
+  manufacturer: string; // Build.MANUFACTURER, e.g. "Xiaomi"
+  model: string; // Build.MODEL — fallback when marketName is empty
 }
 
 interface Entry {
@@ -158,6 +169,31 @@ function tzOffset(d: Date): string {
   return `${mins < 0 ? '-' : '+'}${p(Math.floor(a / 60))}:${p(a % 60)}`;
 }
 
+// The watermark blob HyperOS Gallery reads back (EXIF tag 0x889e) before it
+// will open its watermark editor: without it, a photo edited here fails with
+// "cannot recognize the parameters". Only the stock camera app writes this tag
+// — CameraX/HAL never does — so a camera capture has to synthesize it from the
+// same props the stock app derives it from. Shape matches the stock bytes:
+// {"buildDevice":"aurora",...,"waterLogo":"XIAOMI","waterName":"14 Ultra"}.
+function watermarkBlob(d: DeviceIdentity): string {
+  const brand = d.manufacturer.trim();
+  const market = d.marketName.trim();
+  // "Xiaomi 14 Ultra" -> "14 Ultra": Gallery prints waterName, not the brand.
+  const waterName = market.toLowerCase().startsWith(`${brand.toLowerCase()} `)
+    ? market.slice(brand.length + 1)
+    : market || d.model;
+  return JSON.stringify({
+    buildDevice: d.device,
+    customize: '',
+    cvLensName: '',
+    filterName: '',
+    livephotoInfo: '',
+    version: 2,
+    waterLogo: brand.toUpperCase(),
+    waterName,
+  });
+}
+
 // --- parse -----------------------------------------------------------------
 
 interface ParsedTiff {
@@ -356,6 +392,12 @@ export function writeJpegExif(jpeg: Uint8Array, stamp: ExifStamp, sourceJpeg?: U
     ifd0.set(0x0128, shortEntry(0x0128, 2)); // resolution unit = inch
   }
 
+  // HyperOS Gallery refuses to open its watermark editor without 0x889e, and a
+  // CameraX capture never carries it. Never overwrite a blob the source has.
+  if (stamp.device && !exif.has(0x889e) && /xiaomi|redmi|poco/i.test(stamp.device.manufacturer)) {
+    exif.set(0x889e, asciiEntry(0x889e, watermarkBlob(stamp.device)));
+  }
+
   if (stamp.gps) {
     const { latitude: lat, longitude: lon } = stamp.gps;
     gps.set(0x0001, asciiEntry(0x0001, lat >= 0 ? 'N' : 'S'));