Fix drag-drop review findings: unify teardown, gate FX-add on outside-panel, trim comments

src/shell/panel/panel_state.h

@@ -320,10 +320,8 @@ struct PanelState {
     // when the pointer is not over an FX button.
     MediaTrack* instrumentDropTrack = nullptr;
 
-    // Latched once an OsDrag hand-off attempt for THIS gesture resolves to "cannot hand off"
-    // (empty/unresolvable payload, or the OS isn't ready) — skips re-running
-    // resolveDragPathsForOs (an fs::exists per sample) and the readiness probe on every
-    // subsequent move while the drag stays alive. Cleared by resetDragState.
+    // Latched once an OsDrag hand-off resolves "cannot hand off" for this gesture — skips
+    // re-running resolveDragPathsForOs (fs::exists per sample) each move. Cleared by resetDragState.
     bool dragOsHandoffBlocked = false;
 
     // Authoritative tail setting lives in ReaSamplerSession, not here; panel reads it for