Merge Phase D4: Design View action family + reapply-on-open wiring

tests/test_view_mode_model.cpp

@@ -383,6 +383,30 @@ static void testPlanToggleParkHasEmptyFxOffline() {
     CHECK(plan.park[0].flags.size() == 4);
 }
 
+// -- 8. nextModeId cycle (D4 toggle helper) ----------------------------------
+
+static void testNextModeIdCycles() {
+    ModeRegistry seeded;  // Arrange(0) + Design(1)
+
+    // Two-mode cycle: Arrange -> Design -> Arrange (wraps past the last).
+    CHECK(nextModeId(seeded, kArrangeModeId) == kDesignModeId);
+    CHECK(nextModeId(seeded, kDesignModeId) == kArrangeModeId);
+
+    // Extends to cycle-through-all with >2 modes, in ordinal order.
+    ModeRegistry three;
+    CHECK(three.add(Mode{"mixdown", "Mixdown", 2}));
+    CHECK(nextModeId(three, kArrangeModeId) == kDesignModeId);
+    CHECK(nextModeId(three, kDesignModeId) == "mixdown");
+    CHECK(nextModeId(three, "mixdown") == kArrangeModeId);  // wraps
+
+    // Unknown/stale current id -> first mode (a sane home, not "").
+    CHECK(nextModeId(seeded, "does-not-exist") == kArrangeModeId);
+
+    // Empty registry -> "" (nothing to cycle to).
+    ModeRegistry empty = ModeRegistry::makeEmpty();
+    CHECK(nextModeId(empty, kArrangeModeId).empty());
+}
+
 int main() {
     testNModeRegistryAndMembership();
     testParentDerivationMultiMode();
@@ -393,6 +417,7 @@ int main() {
     testEmptyModelRoundTrip();
     testMalformedJson();
     testPlanToggleParkHasEmptyFxOffline();
+    testNextModeIdCycles();
 
     if (g_fail == 0) std::printf("All tests passed.\n");
     return g_fail ? 1 : 0;