feat(actions): item-level Design/Arrange move + untag actions; W3-A polish

Add three MIDI-bindable item actions mirroring the track tag family, routed
through the shared hookcommand: retag selected items' membership then re-drive
mint/apply so each lands on its mode's managed lane (manual-lane items exempt),
all in one undo block. Extract shared item_read seam, simplify applyMintPlan's
lane-count pass, and guard reconcile against unregistered-mode lanes.

src/view_mode_model.cpp

@@ -158,6 +158,18 @@ std::vector<AutoTag> autoTagNewContent(const std::vector<std::string>& newTrackG
     return tags;
 }
 
+std::vector<ItemRetagOp> planItemRetag(const std::vector<RetagItem>& selected,
+                                       const std::string& targetMode) {
+    std::vector<ItemRetagOp> ops;
+    const bool untag = targetMode.empty();  // empty target ⇒ untag (→ Arrange default)
+    for (const RetagItem& item : selected) {
+        if (item.guid.empty()) continue;    // defensive; a real item always has a GUID
+        if (item.onManualLane) continue;    // manual-lane item is EXEMPT — never retagged
+        ops.push_back(ItemRetagOp{item.guid, untag, untag ? std::string{} : targetMode});
+    }
+    return ops;
+}
+
 // ---------------------------------------------------------------------------
 // lane minting decision
 // ---------------------------------------------------------------------------