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>

test/ctc_decoder_test.dart

@@ -0,0 +1,75 @@
+import 'package:badnote/services/ocr/ctc_decoder.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+/// Build a one-hot-ish logits row of [numClasses] with the max at [maxIndex].
+List<double> _row(int numClasses, int maxIndex) {
+  return List<double>.generate(numClasses, (i) => i == maxIndex ? 1.0 : 0.0);
+}
+
+void main() {
+  group('CtcDecoder.decode', () {
+    test('collapses consecutive repeats and drops blanks (+1 shift)', () {
+      // charset indices: 1->'a', 2->'b', 3->'c' (blank at 0, shifted by one).
+      final decoder = CtcDecoder(['a', 'b', 'c'], blankIndex: 0);
+      const numClasses = 4; // blank + 3 chars
+      final logits = <List<double>>[
+        _row(numClasses, 1), // a
+        _row(numClasses, 1), // a (collapsed)
+        _row(numClasses, 0), // blank
+        _row(numClasses, 2), // b
+        _row(numClasses, 2), // b (collapsed)
+        _row(numClasses, 3), // c
+      ];
+      expect(decoder.decode(logits), 'abc');
+    });
+
+    test('empty input yields empty string', () {
+      final decoder = CtcDecoder(['a', 'b', 'c'], blankIndex: 0);
+      expect(decoder.decode(<List<double>>[]), '');
+    });
+
+    test('all-blank input yields empty string', () {
+      final decoder = CtcDecoder(['a', 'b', 'c'], blankIndex: 0);
+      const numClasses = 4;
+      final logits = <List<double>>[
+        _row(numClasses, 0),
+        _row(numClasses, 0),
+        _row(numClasses, 0),
+      ];
+      expect(decoder.decode(logits), '');
+    });
+
+    test('out-of-range indices are skipped', () {
+      // charset has 2 entries -> valid class indices are 1 and 2. Class index 3
+      // maps to charset[2] which is out of range and must be skipped.
+      final decoder = CtcDecoder(['a', 'b'], blankIndex: 0);
+      const numClasses = 4;
+      final logits = <List<double>>[
+        _row(numClasses, 1), // a
+        _row(numClasses, 3), // out of range -> skipped
+        _row(numClasses, 2), // b
+      ];
+      expect(decoder.decode(logits), 'ab');
+    });
+  });
+
+  group('CtcDecoder.decodeFlat', () {
+    test('reshapes a flat row-major list and decodes it', () {
+      final decoder = CtcDecoder(['a', 'b', 'c'], blankIndex: 0);
+      const numClasses = 4;
+      const timeSteps = 3;
+      final flat = <double>[
+        ..._row(numClasses, 1), // a
+        ..._row(numClasses, 0), // blank
+        ..._row(numClasses, 2), // b
+      ];
+      expect(decoder.decodeFlat(flat, timeSteps, numClasses), 'ab');
+    });
+
+    test('returns empty for non-positive dimensions', () {
+      final decoder = CtcDecoder(['a'], blankIndex: 0);
+      expect(decoder.decodeFlat(<double>[1, 0], 0, 2), '');
+      expect(decoder.decodeFlat(<double>[1, 0], 2, 0), '');
+    });
+  });
+}