Make on-device OCR a pluggable local service so it runs locally on every
platform (not just Windows), aimed at GoodNotes/Notability-class handwriting on
low-power hardware (e.g. Zen2 APU, CPU/iGPU).
- New OcrBackend abstraction (lib/services/ocr/): selector prefers an embedded
ONNX recognition backend, falling back to the OS-native backend (Windows
WinRT), and to a clean no-op when neither is available.
- OnnxRecognitionBackend: flutter_onnxruntime session from a bundled asset,
dart:ui preprocessing (resize to 48px, CHW float32, normalized), pure-Dart CTC
greedy decode. Fully guarded — absent model/dict is a no-op; never throws.
- ocr_engine.dart kept as a thin facade (recognizeImage) delegating to the
selector, so ocr_service.dart is unchanged.
- CtcDecoder unit-tested (6 tests). flutter analyze clean; all tests pass.
- Model is not committed; tool/fetch_ocr_model.sh + assets/models/ocr/README.md
document fetching PP-OCRv4 rec + dict on the dev machine.
- CI: forward HTTPS_PROXY to the Windows build so CMake can fetch the ONNX
Runtime native lib behind the GFW; README documents the system-install
alternative. PP-OCR geometry/blank assumptions documented for on-device tuning.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Akiba So
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