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.h

@@ -0,0 +1,33 @@
+#pragma once
+// view_tree — the ONE genuinely pure piece of the D2 view shell: turning REAPER's
+// linear I_FOLDERDEPTH stream into the parent<->child FolderTree the pure model
+// consumes. The REAPER reads (GetTrack / GetTrackGUID / I_FOLDERDEPTH) stay in
+// view.cpp; this tree arithmetic is REAPER-free so the fiddly folder-depth walk is
+// unit-tested outside the DAW (mirrors capture_paths splitting the path math out).
+//
+// PURE MODULE (CLAUDE.md §load-bearing split): NO REAPER types, NO SWELL, NO
+// vendor/ includes. Standard library + view_mode_model.h (for FolderTree) only.
+
+#include <string>
+#include <vector>
+
+#include "view_mode_model.h"
+
+namespace reasampler {
+
+// One track's contribution to the folder walk, read from REAPER in arrange order.
+// folderDepth is I_FOLDERDEPTH verbatim: 0 = normal, 1 = folder parent (opens a
+// folder after this track), <0 = closes |folderDepth| folder levels after this
+// track (-1 last in innermost, -2 last in innermost + next-innermost, ...).
+struct TrackFolderEntry {
+    std::string guid;
+    int         folderDepth = 0;
+};
+
+// Walks the ordered entries, tracking the open-folder stack, and assigns each
+// node its immediate parentGuid (empty = top level) and isParent (opens a folder).
+// Pure and total: tolerates malformed depth streams (a close deeper than the stack
+// is clamped to empty) so a corrupt/stale project can never fault the shell.
+FolderTree buildFolderTree(const std::vector<TrackFolderEntry>& entries);
+
+} // namespace reasampler