Return every barcode detected in a frame from onReadCode - #821

Summary

Motivation

Some scanning workflows need several codes that are visible at the same time. In our case, a delivery driver scans a location QR (on a fridge) and a shipping-label QR in a single shot.

onReadCode can only deliver one code per frame today, even though both native scanners already detect every code in the frame:

• Android: MLKit BarcodeScanning.process() returns a list, and CKCamera filters it by allowedBarcodeTypes / barcodeFrameSize. onBarcodeRead() then dispatches only barcodes.first().
• iOS: AVCaptureMetadataOutput delivers metadataObjects for every code in the frame, but metadataOutput(_:didOutput:from:) forwards only metadataObjects.first.

The extra codes are computed and then thrown away. This PR forwards all of them. It is a feature addition, not a bug fix.

API change (additive)

onReadCode={(event) => {
  event.nativeEvent.codeStringValue; // unchanged – first code
  event.nativeEvent.codeFormat;      // unchanged – first code
  event.nativeEvent.codes;           // new – [{ codeStringValue, codeFormat }, ...] for every code in the frame
}}

Backward compatibility

The event goes from carrying one code to carrying many, without changing anything existing single-code consumers rely on:

Two points worth a reviewer's call:

• TypeScript: codes is typed as required because native always sends it. Code that only reads the event is unaffected. Code that builds an OnReadCodeData by hand, such as test mocks, needs codes: []. I'm happy to make it optional if you prefer.
• iOS edge case: previously, a frame was dropped entirely if its first metadata object had a nil stringValue. Now that object is skipped, and the remaining valid codes are still reported.

Suggested release: minor.

Implementation

• src/specs/CameraNativeComponent.ts, src/CameraProps.ts: add codes to the event type. This follows the existing onFaceDetected / faces pattern.
• Android: ReadCodeEvent takes a List<ReadCodePayload> and writes codes plus the first code's top-level fields. CKCamera.onBarcodeRead() maps every filtered barcode.
• iOS: the onBarcodeRead closure (CameraProtocol / RealCamera / SimulatorCamera) now passes [ScannedCode]. RealCamera uses compactMap over all metadata objects, and CameraView emits codes alongside the existing fields.
• iOS Fabric: CKCameraViewComponentView.mm builds the codegen OnReadCodeCodes vector, the same way it builds OnFaceDetectedFaces.
• Docs and example: the README onReadCode row and example/src/BarcodeScreenExample.tsx now show codes.

How did you test this change?

Static checks

• tsc --noEmit passes for the library and the example app.
• iOS codegen (react-native codegen --platform ios): the generated EventEmitters.h contains struct OnReadCodeCodes { codeStringValue; codeFormat; } and struct OnReadCode { codeStringValue; codeFormat; std::vector<OnReadCodeCodes> codes; }. These match the designated initializers in CKCameraViewComponentView.mm.

Devices (example app → Barcode screen, two QR codes inside the scan frame)

• iOS (New Arch): two QR codes → codes.length === 2, both values shown
• Android (New Arch): two QR codes → codes.length === 2, both values shown
• Single code on both platforms: codeStringValue / codeFormat unchanged, codes.length === 1
• Old architecture (Paper) build still receives onReadCode