Fix parent visibility: parent visible by own membership OR derived from children

visibleTracks pass 1 now evaluates every node (parent or leaf) by its own
membership, so an untagged folder carrying its own FX/media shows in Arrange
(its default) as well as any mode derived from its children. Adds case-matrix
tests; updates the nested-Arrange assertion to the corrected rule.

src/view_mode_model.h

@@ -150,8 +150,9 @@ private:
 // -- Folder tree (INPUT, not stored) ----------------------------------------
 //
 // The shell builds this from I_FOLDERDEPTH each time and passes it to a visibility
-// query. A node is a leaf or a parent; a parent is visible in every mode any of
-// its descendant leaves belongs to, and is never parked. The master track is
+// query. A node is a leaf or a parent; a parent is visible in a mode if it belongs
+// to that mode by its own membership OR any of its descendant leaves does, and is
+// never parked. The master track is
 // modeled implicitly (always visible, never touched) and is NOT a node here.
 struct FolderNode {
     std::string guid;
@@ -273,8 +274,11 @@ public:
     bool leafBelongsToMode(const std::string& guid, const std::string& modeId) const;
 
     // The set of track GUIDs visible in `modeId`, tree-aware: active leaves,
-    // show-both leaves, and every parent with at least one descendant leaf in the
-    // mode. Untagged leaves count as Arrange. Stale GUIDs in the tree are tolerated.
+    // show-both leaves, and every parent that EITHER belongs to the mode by its own
+    // membership OR has at least one descendant visible in the mode. Untagged nodes
+    // (leaf or folder) count as Arrange, so an untagged folder carrying its own
+    // FX/media shows in Arrange even when none of its children do, and additionally
+    // shows in a child's mode by derivation. Stale GUIDs in the tree are tolerated.
     // The master is not represented (always visible; the shell never touches it).
     std::set<std::string> visibleTracks(const FolderTree& tree,
                                         const std::string& modeId) const;