OCR: embedded, cross-platform ONNX backend with pluggable fallback

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>

lib/services/ocr/ocr_backends.dart

@@ -0,0 +1,45 @@
+import 'dart:typed_data';
+
+import 'native_ocr_backend.dart';
+import 'ocr_backend.dart';
+import 'onnx_recognition_backend.dart';
+
+/// Selects and caches the active local OCR backend.
+///
+/// Preference order: the embedded ONNX recognition backend if its model is
+/// bundled and loads, otherwise the native platform backend, otherwise none.
+class OcrBackends {
+  OcrBackends._();
+
+  static OcrBackend? _active;
+  static bool _resolved = false;
+
+  /// Resolve (once) and return the preferred available backend, or null when
+  /// no backend is available on this device/build.
+  static Future<OcrBackend?> active() async {
+    if (_resolved) return _active;
+
+    final candidates = <OcrBackend>[
+      OnnxRecognitionBackend(),
+      NativeOcrBackend(),
+    ];
+
+    for (final backend in candidates) {
+      if (await backend.isAvailable()) {
+        _active = backend;
+        break;
+      }
+    }
+
+    _resolved = true;
+    return _active;
+  }
+
+  /// Recognize text using the active backend. Returns null when no backend is
+  /// available or nothing was recognized.
+  static Future<String?> recognize(Uint8List png) async {
+    final backend = await active();
+    if (backend == null) return null;
+    return backend.recognize(png);
+  }
+}