Reconcile orphaned Design View snapshots on track delete

Add ViewModeModel::reconcile(liveGuids) to prune snapshots of deleted
tracks; wire it into applyMode before planning. Keep membership so
undo-delete (same GUID) preserves the Design tag.

src/view_mode_model.cpp

@@ -160,6 +160,21 @@ const TrackSnapshot* ViewModeModel::snapshot(const std::string& guid) const {
     return it == snapshots_.end() ? nullptr : &it->second;
 }
 
+std::size_t ViewModeModel::reconcile(const std::set<std::string>& liveGuids) {
+    // Prune snapshots for GUIDs the project no longer contains (see header for the
+    // deliberate snapshot-yes / membership-no asymmetry and the undo-delete rationale).
+    std::size_t removed = 0;
+    for (auto it = snapshots_.begin(); it != snapshots_.end();) {
+        if (liveGuids.count(it->first) == 0) {
+            it = snapshots_.erase(it);
+            ++removed;
+        } else {
+            ++it;
+        }
+    }
+    return removed;
+}
+
 bool ViewModeModel::leafBelongsToMode(const std::string& guid, const std::string& modeId) const {
     const Membership* m = membership_.query(guid);
     if (!m) return modeId == kArrangeModeId;      // untagged ⇒ Arrange default