Make tool-managed lane splits visually transparent: collapse display + lazy-mint

Drive C_LANESCOLLAPSED=2 and hide lane buttons only on tracks the tool itself splits
(gated on the pre-write I_FREEMODE read, so user comp-lane display prefs are never
stomped). Lazy-mint managed lanes only for modes with own content, never an empty
reserved lane; confinement holds via C_LANEPLAYS on the lone lane.

src/view_mode_model.cpp

@@ -220,15 +220,19 @@ LaneMintPlan planLaneMinting(const ViewModeModel& model, const FolderTree& tree,
         // this is the load-bearing "don't lane-split single-mode tracks" rule.
         if (!multiMode) continue;
 
-        // Lanes to mint = the UNION of the modes the own items belong to and the modes the
-        // track is visible in. A folder whose own item is Design-only but which is derived-
-        // visible in Arrange too mints BOTH a Design lane (holding the item) and an Arrange
-        // lane (reserving Arrange's stance slot), matching "each mode owns a fixed lane."
-        std::set<std::string> laneModes = ownItemModes;
-        if (visIt != visibleModesOf.end())
-            laneModes.insert(visIt->second.begin(), visIt->second.end());
+        // Lazy-mint: lanes to mint = ONLY the modes the track's OWN items actually occupy —
+        // never an empty reserved lane for a mode the track is merely derived-visible in.
+        // A folder whose own item is Design-only but which is derived-visible in Arrange too
+        // mints a Design lane ONLY (holding the item); it mints NO Arrange lane. Confinement
+        // still holds: with only a Design lane present, toggling to Arrange drives that lane's
+        // C_LANEPLAYS to 0 (it hides+silences) and no lane plays, so the track reads as an
+        // empty normal track — the Design item does not leak. The Arrange lane is minted on
+        // demand the moment an Arrange item first lands (a later mint tick sees ownItemModes
+        // gain Arrange). The visibility trigger above still decides WHETHER to split; it no
+        // longer inflates WHICH lanes are minted.
+        const std::set<std::string>& laneModes = ownItemModes;
 
-        // Transition to lane-split: one managed lane per involved mode (durable key =
+        // Transition to lane-split: one managed lane per own-content mode (durable key =
         // laneNameForMode(mode)), owned by that mode.
         plan.splits.push_back(LaneMintPlan::TrackSplit{
             track.trackGuid, static_cast<int>(laneModes.size())});