feat(view): mint managed lanes to separate cross-mode content per stance (D2 W3-A)

Pure planLaneMinting decides which tracks hold >1 mode's content and which managed
lane each item lands on; view.cpp applies it (I_FREEMODE/I_NUMFIXEDLANES/P_LANENAME/
I_FIXEDLANE) under one undo block, driven off the auto-tag detection tick. Manual lanes
and their items are never touched. Load-time reconcile rebuilds ownership from durable
lane names before reapplying active-mode visibility.

src/bank_panel.cpp

@@ -753,8 +753,11 @@ void enumerateLiveGuids(ReaProject* proj, std::set<std::string>& allGuids,
 // persist.cpp writes it on the next project save alongside the bank and view state, the
 // same way an action-driven tag is persisted. Wrapping this in an Undo block would flood
 // the REAPER undo history with a new entry for every timer tick that sees new content.
-void detectNewContent() {
-    if (!g_panel.session) return;
+// Returns true iff this tick tagged at least one new GUID into a mode — the signal the
+// caller uses to decide whether to run the lane-minting pass (a track can only newly
+// become multi-mode when auto-tag just placed content on it). No tag ⇒ nothing to mint.
+bool detectNewContent() {
+    if (!g_panel.session) return false;
 
     ReaProject* proj = EnumProjects(-1, nullptr, 0);
 
@@ -778,7 +781,7 @@ void detectNewContent() {
     enumerateLiveGuids(proj, live, itemOnManualLane);
 
     const std::vector<std::string> added = g_panel.contentBaseline.observe(live);
-    if (added.empty()) return;  // first poll after open, or nothing new this tick
+    if (added.empty()) return false;  // first poll after open, or nothing new this tick
 
     // Split the new GUIDs into tracks vs items so the pure decision can apply the
     // manual-lane exemption to items only. A GUID present in the item-lane map is an
@@ -799,6 +802,7 @@ void detectNewContent() {
         autoTagNewContent(newTracks, newItems, model.activeModeId());
     for (const AutoTag& tag : tags)
         model.membership().tag(tag.guid, tag.modeId);
+    return !tags.empty();
 }
 
 // --- Audition preview ---------------------------------------------------------
@@ -1245,7 +1249,18 @@ void bankPanelRefresh() {
     // tracks/items are created in the arrange view, not the panel, so detection must
     // not be gated on the dock being visible. READ-ONLY on the project; only mutates
     // the in-memory membership index (persist saves it like any action-driven tag).
-    detectNewContent();
+    const bool tagged = detectNewContent();
+
+    // Lane minting (D2 Wave 3) runs ONLY when detection just tagged new content — a
+    // track can only newly become multi-mode when auto-tag placed content on it. Unlike
+    // the invisible membership tag above, minting is a visible structural mutation
+    // (I_FREEMODE/I_FIXEDLANE/P_LANENAME), so mintManagedLanes wraps it in its own Undo
+    // block and only mints for tracks that hold >1 mode's content — a single-mode track
+    // is left to D1 whole-track parking. Managed lanes only; manual lanes untouched.
+    if (tagged && g_panel.session) {
+        ReaProject* proj = EnumProjects(-1, nullptr, 0);
+        mintManagedLanes(g_panel.session->view(), proj);
+    }
 
     if (!g_panel.open || !g_panel.hwnd) return;
     // Repaint only when the bank actually changed (generation bump). Cheap tick