feat(view): D4 Design View action family + reapply-on-open wiring

Register toggle/activate/tag/untag/show-both actions driving the D2
shell and D3 model; reapply saved active mode on project load via a
persist load-signal seam. Shared guidString helper; pure nextModeId
unit-tested.

src/persist.h

@@ -89,6 +89,18 @@ public:
     // Intended to be driven by REAPER's "timer" register. Idempotent per tick.
     void poll();
 
+    // Load signal for the D4 reapply-on-open glue. poll() raises this whenever it
+    // (re)loads the view model from a project — prime, a project switch/open, or a
+    // forked-sibling load. consumeLoadSignal() returns true ONCE per load and clears
+    // it, so the integration layer (main.cpp) can react by reapplying the saved
+    // active mode's visibility exactly once, then goes quiet on idle ticks.
+    //
+    // Signal-based seam by design: persist stays MODEL-ONLY (it never calls the view
+    // shell), so there is no persist -> view dependency. main.cpp owns the glue —
+    // it drives both persist.poll() and view::applyMode, so the reapply wiring lives
+    // where those two already meet. D3 deliberately deferred exactly this to D4.
+    bool consumeLoadSignal();
+
 private:
     BankIndex bank_;
 
@@ -108,6 +120,7 @@ private:
     std::string lastGuid_;                // "" until the first saved project is seen
     std::string lastRppPath_;             // .rpp path last seen for lastProject_
     bool        primed_ = false;          // false until the first poll() observes state
+    bool        loadPending_ = false;     // raised by loadFromProject; drained by consumeLoadSignal
 
     // Load the index from the given project's ext state and resolve bank paths
     // against projectDir. Replaces the in-memory bank. projectDir empty -> clears