feat(exif): stamp RC_<ts> filenames + EXIF on capture and export

src/utils/photoMeta.ts

@@ -0,0 +1,47 @@
+// Gallery-facing photo metadata: the RC_ filename convention and the file-level
+// EXIF helpers both export engines share (the codec itself lives in exifWrite.ts).
+import { File } from 'expo-file-system';
+import * as FileSystem from 'expo-file-system/legacy';
+import { writeJpegExif, SOFTWARE, type ExifStamp } from './exifWrite';
+
+// RC_<yyyymmdd_hhmmss>[_exported].jpg — the name the gallery shows for a saved
+// photo. The exported variant marks a photo re-exported from the library.
+export function rcPhotoName(exported: boolean, date = new Date()): string {
+  const p = (n: number) => String(n).padStart(2, '0');
+  const stamp = `${date.getFullYear()}${p(date.getMonth() + 1)}${p(date.getDate())}_${p(date.getHours())}${p(date.getMinutes())}${p(date.getSeconds())}`;
+  return `RC_${stamp}${exported ? '_exported' : ''}.jpg`;
+}
+
+// Cache path for the next export. Two renders landing in the same second would
+// collide on the name AND on RN's <Image> cache (keyed by URI), so the
+// timestamp walks forward a second at a time instead of taking a suffix.
+export function nextPhotoPath(exported: boolean): string {
+  let ms = Date.now();
+  for (;;) {
+    const path = `${FileSystem.cacheDirectory}${rcPhotoName(exported, new Date(ms))}`;
+    if (!new File(path).exists) return path;
+    ms += 1000;
+  }
+}
+
+export async function readPhotoBytes(uri: string): Promise<Uint8Array | null> {
+  try {
+    return await new File(uri).bytes();
+  } catch (e) {
+    console.warn('EXIF source read failed:', e);
+    return null;
+  }
+}
+
+// Stamp EXIF into a file already on disk (the native engine writes the JPEG
+// itself, so those bytes never pass through JS). Never throws — a metadata
+// failure must not cost the user the photo.
+export async function stampExifOnFile(fileUri: string, stamp: ExifStamp, sourceUri?: string | null): Promise<void> {
+  try {
+    const target = new File(fileUri);
+    const source = sourceUri ? await readPhotoBytes(sourceUri) : null;
+    target.write(writeJpegExif(await target.bytes(), { software: SOFTWARE, ...stamp }, source));
+  } catch (e) {
+    console.warn('EXIF stamp failed:', e);
+  }
+}