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.h

@@ -267,6 +267,26 @@ public:
     const TrackSnapshot* snapshot(const std::string& guid) const;
     const std::map<std::string, TrackSnapshot>& snapshots() const { return snapshots_; }
 
+    // Prunes orphaned per-track state: drops every snapshot whose GUID is NOT in
+    // `liveGuids` (the set of GUIDs the shell currently enumerates from the project).
+    // Returns the number of snapshots removed. The shell calls this before planning a
+    // toggle; because reapply-on-load also routes through the shell's applyMode, this
+    // reconciles on project open too.
+    //
+    // Why snapshots and NOT membership: a parked track's snapshot is dead weight once
+    // the track is deleted — it can never be restored, and if REAPER reuses that GUID
+    // for a different track a stale snapshot would drive an INCORRECT restore. So it
+    // must be pruned. Membership is deliberately KEPT: REAPER's undo of a track delete
+    // restores the SAME GUID, so dropping the Design tag on delete would silently lose
+    // it on undo-delete. Keeping membership means an undone delete brings the track
+    // back correctly tagged and it re-snapshots + re-parks cleanly on the next toggle.
+    // A genuinely-deleted-and-never-restored track leaves only a tiny dormant
+    // membership entry — acceptable, and far better than losing tags on undo. Folder
+    // RESTRUCTURE (moving tracks without deleting) is already self-healing: the tree is
+    // rebuilt from I_FOLDERDEPTH every toggle, so a restructure leaves every GUID live
+    // and reconcile is a no-op over it. This handles DELETION specifically.
+    std::size_t reconcile(const std::set<std::string>& liveGuids);
+
     // Does `guid` belong to `modeId`? A leaf belongs if it is tagged into modeId,
     // is show-both (belongs everywhere), or is untagged and modeId is Arrange (the
     // default). Parent derivation is NOT applied here — this is the LEAF rule; use