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_engine.dart

@@ -1,21 +1,14 @@
-import 'dart:io';
+import 'dart:typed_data';
 
-import 'package:flutter/services.dart';
+import 'ocr/ocr_backends.dart';
 
-/// Platform OCR backend. Uses Windows built-in OCR on desktop Windows.
+/// Local OCR entry point. Delegates to a pluggable backend (embedded ONNX
+/// recognition when a model is bundled, otherwise the native platform OCR).
+///
+/// The static API is kept for back-compat with [OcrService].
 class OcrEngine {
-  static const _channel = MethodChannel('badnote/ocr');
-
   /// Recognize text from a PNG image. Returns null when unavailable or empty.
-  static Future<String?> recognizeImage(Uint8List pngBytes) async {
-    if (!Platform.isWindows) return null;
-    try {
-      final result = await _channel.invokeMethod<String>('recognize', pngBytes);
-      final text = result?.trim();
-      if (text == null || text.isEmpty) return null;
-      return text;
-    } catch (_) {
-      return null;
-    }
+  static Future<String?> recognizeImage(Uint8List pngBytes) {
+    return OcrBackends.recognize(pngBytes);
   }
 }