fix(4x3): force frame stream to 4:3 sensor aspect via targetResolution

src/components/Viewfinder.tsx

@@ -155,6 +155,35 @@ export default function Viewfinder({
   // enough to pick the device up — no manual polling needed.
   const device = useCameraDevice('back');
 
+  // CameraX negotiates the CameraFrameOutput aspect on its own (default 16:9 on
+  // this device), which center-crops the 4:3 sensor — so the ratio band could
+  // never display 100% of the sensor pixels. Force the frame stream to a 4:3
+  // (sensor aspect) resolution supported for 'stream' outputs; the photo output
+  // keeps its own full-res 4:3 size and FOV, so the viewfinder then matches the
+  // captured photo exactly. Best supported 4:3 size closest to 1280x960
+  // (720p-class, keeps the frame pipeline light); CameraX may still round it.
+  const frameTarget = useMemo(() => {
+    if (!device) return undefined;
+    try {
+      let best: { width: number; height: number } | undefined;
+      let bestScore = Infinity;
+      for (const s of device.getSupportedResolutions('stream')) {
+        const w = s.width;
+        const h = s.height;
+        if (!(w > 0 && h > 0)) continue;
+        if (Math.abs(w / h - 4 / 3) > 0.03) continue;
+        const score = Math.abs(Math.sqrt(w * h) - Math.sqrt(1280 * 960));
+        if (score < bestScore) {
+          bestScore = score;
+          best = { width: w, height: h };
+        }
+      }
+      return best ?? undefined;
+    } catch {
+      return undefined;
+    }
+  }, [device]);
+
   const skiaImage = useImage(libraryImageUri || '');
   // Wall-frame PNG artwork (project root). Loaded lazily so other frames pay
   // no asset cost; source switches to '' when unselected (useImage → null).
@@ -629,7 +658,8 @@ export default function Viewfinder({
       ' ratio=' + aspectRatio +
       ' frame=' + selectedFrame +
       ' view=' + vw + 'x' + vh +
-      ' feed=' + (cameraPhotoRect ? JSON.stringify([1, vw / vh, cameraPhotoRect.x / vw, cameraPhotoRect.y / vh, cameraPhotoRect.w / vw, cameraPhotoRect.h / vh]) : 'null')
+      ' feed=' + (cameraPhotoRect ? JSON.stringify([1, vw / vh, cameraPhotoRect.x / vw, cameraPhotoRect.y / vh, cameraPhotoRect.w / vw, cameraPhotoRect.h / vh]) : 'null') +
+      ' frameTarget=' + (frameTarget ? frameTarget.width + 'x' + frameTarget.height : 'null')
     );
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [region, regionBand, aspectRatio, selectedFrame, cameraPhotoRect]);
@@ -956,6 +986,7 @@ export default function Viewfinder({
           isActive={appActive}
           outputs={cameraOutputs}
           pixelFormat="yuv"
+          targetResolution={frameTarget}
           onStarted={() => setCameraActive(true)}
           onFrame={handleFrame}
           onError={onSessionError}