feat(view): D2 view shell — apply mode toggle plan to live project

Reads I_FOLDERDEPTH into a FolderTree (pure view_tree helper, unit-tested),
snapshots to-be-parked tracks before parking, runs the D1 planner, and applies
park/restore flag + per-FX-offline writes under an undo block. Master and
mute/solo untouched; acts only on tagged tracks.

src/view_tree.cpp

@@ -0,0 +1,41 @@
+// view_tree — pure folder-depth walk. See view_tree.h.
+
+#include "view_tree.h"
+
+namespace reasampler {
+
+FolderTree buildFolderTree(const std::vector<TrackFolderEntry>& entries) {
+    FolderTree tree;
+    tree.nodes.reserve(entries.size());
+
+    // Stack of currently-open folder-parent GUIDs. The top is the immediate parent
+    // of the next track. A folder-parent track opens its folder AFTER contributing
+    // its own node (its own parent is the enclosing folder), so the push trails the
+    // assignment. A closing track belongs to the folder it closes, so the pop also
+    // trails the assignment.
+    std::vector<std::string> open;
+
+    for (const TrackFolderEntry& e : entries) {
+        FolderNode node;
+        node.guid       = e.guid;
+        node.parentGuid = open.empty() ? std::string{} : open.back();
+        node.isParent   = e.folderDepth == 1;
+        tree.nodes.push_back(node);
+
+        if (e.folderDepth == 1) {
+            open.push_back(e.guid);  // this track's folder opens for what follows
+        } else if (e.folderDepth < 0) {
+            // Closes |folderDepth| levels after this (already-assigned) track.
+            // Clamp to the stack size so a malformed/stale depth stream can't
+            // underflow — the walk stays total.
+            int levels = -e.folderDepth;
+            while (levels-- > 0 && !open.empty()) {
+                open.pop_back();
+            }
+        }
+    }
+
+    return tree;
+}
+
+} // namespace reasampler