feat(storage): sidecar model + atomic store (lib only)

Phase 1 of the file-based storage plan. Pure library, no runtime
behavior change yet (editors still use SQLite).

- BadnoteSidecar: per-file annotation document (schema-versioned)
  holding per-page ink (EditorStroke JSON), text highlights,
  scratch-link anchors (ScratchLink JSON) each with its own
  scratchpad (InkStroke world-coord JSON), and bookmarks. Reuses the
  existing toJson formats — no parallel stroke format.
- SidecarStore.writeAtomic: temp-file + rename atomic write keeping a
  .bak; read() falls back to .bak on a missing/corrupt primary.

Round-trip + atomic-write + .bak-recovery tests. analyze clean,
322 tests green.

test/badnote_sidecar_test.dart

@@ -0,0 +1,212 @@
+// test/badnote_sidecar_test.dart
+//
+// Round-trips a BadnoteSidecar containing per-page strokes + highlights +
+// bookmarks + scratch-links (each with its own scratchpad of InkStrokes),
+// asserting the re-parsed model equals the original. Verifies the sidecar reuses
+// the existing EditorStroke / InkStroke / ScratchLink / Bookmark JSON shapes and
+// that the schema `version` field survives.
+
+import 'dart:convert';
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/engine/stroke_model.dart';
+import 'package:badnote/models/bookmark.dart';
+import 'package:badnote/models/ink_point.dart';
+import 'package:badnote/models/ink_stroke.dart';
+import 'package:badnote/models/pen_tool.dart';
+import 'package:badnote/models/pointer_device_kind.dart';
+import 'package:badnote/models/scratch_link.dart';
+import 'package:badnote/storage/badnote_sidecar.dart';
+
+EditorStroke _editorStroke(String id, EditorTool tool) => EditorStroke(
+      id: id,
+      points: [
+        const EditorPoint(x: 0.1, y: 0.2, pressure: 0.5, tilt: 0.1),
+        EditorPoint(
+          x: 0.3,
+          y: 0.4,
+          pressure: 0.9,
+          tilt: 0.2,
+          timestamp: 1234,
+          pointerDeviceKind: InputDeviceKind.stylus,
+        ),
+      ],
+      tool: tool,
+      color: 0xFF112233,
+      width: 0.005,
+    );
+
+InkStroke _inkStroke(String id) => InkStroke(
+      id: id,
+      points: [
+        const InkPoint(
+          x: 100.0,
+          y: 200.0,
+          pressure: 0.7,
+          tilt: 0.3,
+          timestamp: 99,
+          pointerDeviceKind: InputDeviceKind.stylus,
+        ),
+        const InkPoint(x: 300.0, y: 400.0, timestamp: 100),
+      ],
+      tool: PenTool.pen,
+      color: 0xFF445566,
+      strokeWidth: 3.0,
+      createdAt: DateTime.utc(2026, 6, 24, 10, 0, 0),
+    );
+
+BadnoteSidecar _fullSidecar() => BadnoteSidecar(
+      sourceFile: 'Calculus Lecture 3.pdf',
+      docType: 'pdf',
+      pageCount: 42,
+      rotation: 90,
+      createdAt: DateTime.utc(2026, 6, 24, 10, 0, 0),
+      updatedAt: DateTime.utc(2026, 6, 24, 10, 32, 11),
+      strokes: {
+        0: [_editorStroke('s0', EditorTool.pen)],
+        3: [
+          _editorStroke('s1', EditorTool.highlighter),
+          _editorStroke('s2', EditorTool.pen),
+        ],
+      },
+      highlights: {
+        0: const [
+          SidecarHighlight(l: 0.12, t: 0.20, r: 0.88, b: 0.235, color: 0xFFFFEB3B),
+        ],
+      },
+      bookmarks: [
+        Bookmark(
+          id: 'bm1',
+          documentId: 'doc1',
+          pageNumber: 5,
+          label: 'Proof',
+          color: 0xFF2196F3,
+          createdAt: DateTime.utc(2026, 6, 24, 9, 0, 0),
+        ),
+      ],
+      scratchLinks: [
+        SidecarScratchLink(
+          link: const ScratchLink(
+            id: 'anchor1',
+            documentId: 'doc1',
+            pageIndex: 7,
+            nx: 0.83,
+            ny: 0.41,
+          ),
+          scratchpad: SidecarScratchpad(
+            canvasWidth: 5000,
+            canvasHeight: 6000,
+            strokes: [_inkStroke('ink1'), _inkStroke('ink2')],
+          ),
+        ),
+      ],
+    );
+
+/// Serializes through `jsonEncode`/`jsonDecode` exactly as `SidecarStore`
+/// persists/loads on disk (nested freezed objects only flatten via the encoder's
+/// toEncodable hook, so an in-memory map round-trip would not).
+BadnoteSidecar _roundTrip(BadnoteSidecar sidecar) => BadnoteSidecar.fromJson(
+    jsonDecode(jsonEncode(sidecar.toJson())) as Map<String, dynamic>);
+
+void main() {
+  test('full round-trip preserves strokes/highlights/bookmarks/links', () {
+    final original = _fullSidecar();
+    final reparsed = _roundTrip(original);
+
+    expect(reparsed.version, kBadnoteSidecarVersion);
+    expect(reparsed.sourceFile, 'Calculus Lecture 3.pdf');
+    expect(reparsed.docType, 'pdf');
+    expect(reparsed.pageCount, 42);
+    expect(reparsed.rotation, 90);
+    expect(reparsed.createdAt, DateTime.utc(2026, 6, 24, 10, 0, 0));
+    expect(reparsed.updatedAt, DateTime.utc(2026, 6, 24, 10, 32, 11));
+
+    // Strokes (EditorStroke value equality via freezed).
+    expect(reparsed.strokes.keys.toSet(), {0, 3});
+    expect(reparsed.strokes[0], original.strokes[0]);
+    expect(reparsed.strokes[3], original.strokes[3]);
+
+    // Highlights.
+    expect(reparsed.highlights[0], original.highlights[0]);
+
+    // Bookmarks (Bookmark value equality via freezed).
+    expect(reparsed.bookmarks, original.bookmarks);
+
+    // Scratch links + embedded scratchpads.
+    expect(reparsed.scratchLinks, original.scratchLinks);
+    final sp = reparsed.scratchLinks.single.scratchpad;
+    expect(sp.canvasWidth, 5000);
+    expect(sp.canvasHeight, 6000);
+    expect(sp.strokes, original.scratchLinks.single.scratchpad.strokes);
+  });
+
+  test('strokes JSON is byte-compatible with EditorStroke.toJson', () {
+    final stroke = _editorStroke('s0', EditorTool.pen);
+    final sidecar = BadnoteSidecar(strokes: {2: [stroke]});
+    final json = sidecar.toJson();
+    final pageList = (json['strokes'] as Map)['2'] as List;
+    expect(pageList.single, stroke.toJson());
+  });
+
+  test('scratchpad strokes JSON is byte-compatible with InkStroke.toJson', () {
+    final ink = _inkStroke('ink1');
+    final scratchpad = SidecarScratchpad(strokes: [ink]);
+    final json = scratchpad.toJson();
+    expect((json['strokes'] as List).single, ink.toJson());
+  });
+
+  test('scratch link JSON includes ScratchLink fields plus scratchpad', () {
+    const link = ScratchLink(
+      id: 'a',
+      documentId: 'd',
+      pageIndex: 3,
+      nx: 0.5,
+      ny: 0.5,
+    );
+    final json = SidecarScratchLink(link: link).toJson();
+    // Reuses ScratchLink.toJson keys verbatim.
+    for (final key in link.toJson().keys) {
+      expect(json.containsKey(key), isTrue, reason: 'missing key $key');
+    }
+    expect(json.containsKey('scratchpad'), isTrue);
+  });
+
+  test('empty sidecar round-trips to empty containers', () {
+    final reparsed = _roundTrip(BadnoteSidecar());
+    expect(reparsed.strokes, isEmpty);
+    expect(reparsed.highlights, isEmpty);
+    expect(reparsed.bookmarks, isEmpty);
+    expect(reparsed.scratchLinks, isEmpty);
+    expect(reparsed.version, kBadnoteSidecarVersion);
+  });
+
+  test('unknown fields are ignored (forward-compat)', () {
+    final encoded = jsonEncode(BadnoteSidecar(
+      strokes: {0: [_editorStroke('s0', EditorTool.pen)]},
+    ).toJson());
+    final json = jsonDecode(encoded) as Map<String, dynamic>;
+    json['futureFieldWeDoNotKnow'] = {'anything': true};
+    ((json['strokes'] as Map)['0'] as List)[0]['brush'] =
+        'marker'; // future per-stroke field
+    final reparsed = BadnoteSidecar.fromJson(json);
+    expect(reparsed.strokes[0]!.single.id, 's0');
+  });
+
+  test('missing scratchpad defaults to 4000x4000 empty pad', () {
+    final json = SidecarScratchLink(
+      link: const ScratchLink(
+        id: 'a',
+        documentId: 'd',
+        pageIndex: 0,
+        nx: 0,
+        ny: 0,
+      ),
+    ).toJson();
+    json.remove('scratchpad');
+    final reparsed = SidecarScratchLink.fromJson(json);
+    expect(reparsed.scratchpad.canvasWidth, 4000.0);
+    expect(reparsed.scratchpad.canvasHeight, 4000.0);
+    expect(reparsed.scratchpad.strokes, isEmpty);
+  });
+}