feat(storage): PDF editor persists to per-file sidecar

Phase 2 (core swap). The PDF editor and split-view scratchpad stop
writing SQLite and persist to a per-file sidecar
`<pdfPath>.badnote.json` (debounced, atomic temp+rename+.bak) — so
annotations travel with the file. The source path is the identity
(no more djb2 doc-id).

- SidecarRepository wraps the Phase-1 store with debounced autosave.
- pen_editor: per-page ink, scratch-links AND highlights now persist
  to the sidecar and restore on reopen (closes persist-highlights).
- New "un-highlight" tool: tap a stored highlight to remove it — the
  highlight could not be removed before.
- split_view: each anchor's scratchpad lives in the sidecar's
  scratchLinks[id].scratchpad, keyed by anchor id.

Note: pre-existing SQLite annotations are migrated later (Phase 5);
note/slide editors swap in Phase 4. analyze clean, tests green.

lib/screens/split_view_screen.dart

@@ -13,7 +13,6 @@
 // ToolButton / color dots), replacing the old AnnotationToolbar.
 
 import 'dart:async';
-import 'dart:convert';
 
 import 'package:flutter/material.dart';
 import 'package:pdfrx/pdfrx.dart';
@@ -25,21 +24,22 @@ import '../editor/canvas/pen_stroke.dart';
 import '../editor/engine/brush.dart';
 import '../editor/input/pen_config.dart' show kDefaultEraserRadius;
 import '../editor/notebook/ink_stroke_adapter.dart';
+import '../editor/persistence/sidecar_repository.dart';
 import '../models/ink_stroke.dart';
-import '../services/database_service.dart';
 import '../services/undo_manager.dart';
+import '../storage/badnote_sidecar.dart';
 
 /// Split-view derivation surface for a single scratch-link anchor: left pane =
 /// the reference PDF (at [initialPage]), right pane = the anchor's private
-/// infinite scratchpad. Scratchpad strokes persist per ANCHOR via
-/// [DatabaseService.saveScratchpad] / [DatabaseService.loadScratchpad], keyed by
-/// [scratchLinkId].
+/// infinite scratchpad. Scratchpad strokes persist inside the source file's
+/// SIDECAR (`<filePath>.badnote.json`), embedded in the anchor's
+/// `scratchLinks[id].scratchpad` (keyed by [scratchLinkId]). Strokes keep the
+/// absolute world-pixel [InkStroke] format unchanged.
 class SplitViewScreen extends StatefulWidget {
   final String filePath;
-  final String documentId;
 
-  /// The owning anchor id. Doubles as the scratchpad storage key so this is the
-  /// anchor's private scratch space.
+  /// The owning anchor id. Selects which `scratchLinks[].scratchpad` in the
+  /// sidecar this is the private scratch space for.
   final String scratchLinkId;
 
   /// 0-based page the anchor sits on; the left PDF opens here.
@@ -48,7 +48,6 @@ class SplitViewScreen extends StatefulWidget {
   const SplitViewScreen({
     super.key,
     required this.filePath,
-    required this.documentId,
     required this.scratchLinkId,
     this.initialPage = 0,
   });
@@ -102,6 +101,11 @@ class _SplitViewState extends State<SplitViewScreen> {
   Timer? _saveTimer;
   bool _dirty = false;
 
+  /// Sidecar persistence for the source file (the scratchpad is embedded in the
+  /// anchor's `scratchLinks[id].scratchpad`). Null until [_loadScratchpad]
+  /// resolves.
+  SidecarRepository? _repo;
+
   static const double _edgeThreshold = 200.0;
   static const double _expandAmount = 1000.0;
 
@@ -116,18 +120,34 @@ class _SplitViewState extends State<SplitViewScreen> {
   void dispose() {
     _saveTimer?.cancel();
     _saveImmediate();
+    final repo = _repo;
+    if (repo != null) {
+      repo.flush(); // fire-and-forget; atomic write finishes off the tree
+      repo.dispose();
+    }
     // PdfViewerController (pdfrx) has no dispose(); it detaches with the viewer.
     _scratchTransform.dispose();
     super.dispose();
   }
 
-  // -- Persistence (keyed by the ANCHOR id, not the documentId) --
+  // -- Persistence (sidecar's scratchLinks[id].scratchpad, keyed by anchor id) --
 
   Future<void> _loadScratchpad() async {
-    final db = await DatabaseService.getInstance();
-    final strokes = await db.loadScratchpad(widget.scratchLinkId);
-    if (!mounted) return;
+    final repo = await SidecarRepository.open(widget.filePath, docType: 'pdf');
+    if (!mounted) {
+      repo.dispose();
+      return;
+    }
+    _repo = repo;
+    final pad = repo.scratchpadFor(widget.scratchLinkId);
     setState(() {
+      if (pad != null) {
+        // Restore the world size so the infinite canvas reopens at its grown
+        // extent (previously always reset to 4000×4000).
+        _canvasWidth = pad.canvasWidth;
+        _canvasHeight = pad.canvasHeight;
+      }
+      final strokes = pad?.strokes ?? const <InkStroke>[];
       // Keep only freehand strokes so the canvas list stays 1:1 with the undo
       // manager (shapes/text have no pen-canvas representation).
       final freehand = strokes.where((s) => isFreehandTool(s.tool)).toList();
@@ -147,9 +167,17 @@ class _SplitViewState extends State<SplitViewScreen> {
   Future<void> _saveImmediate() async {
     if (!_dirty) return;
     _dirty = false;
-    final db = await DatabaseService.getInstance();
-    final json = jsonEncode(_strokes.map((s) => s.toJson()).toList());
-    await db.saveScratchpad(widget.scratchLinkId, json);
+    final repo = _repo;
+    if (repo == null) return;
+    repo.scheduleScratchpadSave(
+      widget.scratchLinkId,
+      SidecarScratchpad(
+        canvasWidth: _canvasWidth,
+        canvasHeight: _canvasHeight,
+        strokes: List<InkStroke>.of(_strokes),
+      ),
+    );
+    await repo.flush();
   }
 
   // -- Scratchpad stroke callbacks --