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.h

@@ -0,0 +1,50 @@
+#pragma once
+// view — the REAPER-facing shell of the Design View feature (Phase D2). It is the
+// mirror of the capture shell: the ViewModeModel (pure, D1) holds the mode/
+// membership/snapshot state and emits the toggle plan; this shell reads the live
+// project's folder tree, snapshots the tracks it is about to park, runs the model's
+// planner, and applies the resulting flag + per-FX-offline writes to REAPER.
+//
+// It includes view_mode_model (pure) but NO REAPER headers — the .cpp is the one
+// REAPER-facing translation unit (CLAUDE.md §contract: only main.cpp defines the
+// API pointers; every other .cpp gets them extern). Callers (persist, actions)
+// depend on this seam without dragging the SDK into their include sites.
+//
+// Hard invariants this shell enforces (CONTEXT.md §Design View, precision
+// invariants) — verified in self-review, never crossed:
+//   * Never touches the master track's visibility (SDK forbids B_SHOWINTCP/
+//     B_SHOWINMIXER on master); the master is never a node in the tree.
+//   * Never reads or writes B_MUTE / I_SOLO on any track.
+//   * Acts only on tracks the model owns (tagged leaves the planner names) plus
+//     derived parents' visibility — never an untagged track's owned flags.
+//   * Snapshots every to-be-parked track's prior flags BEFORE parking, storing
+//     them into the model so restore is faithful and survives a save-while-parked.
+
+#include <string>
+
+#include "view_mode_model.h"
+
+// REAPER's opaque project handle. Forward-declared to keep this header SDK-free;
+// the .cpp includes reaper_plugin_functions.h and sees the real class.
+class ReaProject;
+
+namespace reasampler {
+
+// Applies `targetModeId` to the live project `proj`:
+//   1. Reads the arrange-ordered track list, builds the FolderTree from
+//      I_FOLDERDEPTH (via the pure buildFolderTree helper).
+//   2. Runs model.planToggle(tree, targetModeId).
+//   3. For each track about to be PARKED: snapshots its current B_SHOWINTCP /
+//      B_SHOWINMIXER / B_MAINSEND / I_FXEN and per-FX offline state, stores the
+//      snapshot into the model, THEN applies the park writes (expanding the
+//      per-FX offline loop from TrackFX_GetCount, which the pure plan leaves empty).
+//   4. For each track to RESTORE: applies the plan's snapshot-sourced flag + per-FX
+//      offline writes verbatim.
+//   5. Sets the model's active mode to `targetModeId`.
+// All track mutations are wrapped in Undo_BeginBlock2 / Undo_EndBlock2.
+//
+// Returns false (no mutation, active mode unchanged) if `targetModeId` is not a
+// registered mode. `proj` may be nullptr to mean REAPER's current project.
+bool applyMode(ViewModeModel& model, const std::string& targetModeId, ReaProject* proj);
+
+} // namespace reasampler