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>

macos/Flutter/GeneratedPluginRegistrant.swift

@@ -8,6 +8,7 @@ import Foundation
 import device_info_plus
 import file_picker
 import file_selector_macos
+import flutter_onnxruntime
 import shared_preferences_foundation
 import sqflite_darwin
 import syncfusion_pdfviewer_macos
@@ -17,6 +18,7 @@ func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
   DeviceInfoPlusMacosPlugin.register(with: registry.registrar(forPlugin: "DeviceInfoPlusMacosPlugin"))
   FilePickerPlugin.register(with: registry.registrar(forPlugin: "FilePickerPlugin"))
   FileSelectorPlugin.register(with: registry.registrar(forPlugin: "FileSelectorPlugin"))
+  FlutterOnnxruntimePlugin.register(with: registry.registrar(forPlugin: "FlutterOnnxruntimePlugin"))
   SharedPreferencesPlugin.register(with: registry.registrar(forPlugin: "SharedPreferencesPlugin"))
   SqflitePlugin.register(with: registry.registrar(forPlugin: "SqflitePlugin"))
   SyncfusionFlutterPdfViewerPlugin.register(with: registry.registrar(forPlugin: "SyncfusionFlutterPdfViewerPlugin"))