fix: Surface pen pressure, zoom glitches, sticky notes, selection UX

Wire Win32 pressure into Dart, tighten pinch guards, use geometric shape
strokes, expand the ink palette, and replace scratch-link split view with
an on-page sticky that shares the sidecar repo.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/editor/persistence/sidecar_repository.dart

@@ -99,6 +99,11 @@ class SidecarRepository {
   Timer? _timer;
   bool _disposed = false;
 
+  /// How many editors currently hold this repo. [open] reuses an existing
+  /// instance and bumps the count; [dispose] only tears down at zero so a
+  /// split-view / sticky overlay cannot clobber the PDF editor's sidecar.
+  int _retainCount = 1;
+
   /// Tail of the in-flight write chain. Writes are serialized through this so a
   /// debounce-timer write and a concurrent lifecycle [flush] can't race on the
   /// same `.tmp`/rename (which would throw on the loser). Each write always
@@ -107,14 +112,23 @@ class SidecarRepository {
 
   /// Open (or create) the repository for [sourceFilePath]. Reads the existing
   /// sidecar if present (falling back to its `.bak`), else starts empty.
+  ///
+  /// Reuses an already-open repo for the same path (retain-counted) so a
+  /// scratchpad overlay / split view cannot race the PDF editor with a second
+  /// in-memory snapshot that would overwrite scratchpad ink on flush.
   static Future<SidecarRepository> open(
     String sourceFilePath, {
     String? docType,
     Duration debounce = const Duration(milliseconds: 800),
   }) async {
+    final existing = SidecarRepositoryRegistry.forPath(sourceFilePath);
+    if (existing != null && !existing._disposed) {
+      existing._retainCount++;
+      return existing;
+    }
     final file = File('$sourceFilePath$kSidecarSuffix');
-    final existing = await SidecarStore.read(file);
-    final sidecar = existing ??
+    final loaded = await SidecarStore.read(file);
+    final sidecar = loaded ??
         BadnoteSidecar(
           sourceFile: _basename(sourceFilePath),
           docType: docType,
@@ -313,6 +327,11 @@ class SidecarRepository {
 
   /// Cancel pending timers. Call [flush] first to persist pending writes.
   void dispose() {
+    if (_disposed) return;
+    if (_retainCount > 1) {
+      _retainCount--;
+      return;
+    }
     _disposed = true;
     _timer?.cancel();
     _timer = null;