dashboard: clearer label for the dropped-NC detail rows

"dropped detail:" said nothing about what those pins are. They were detached
by drop_singleton_signals because each was the lone pin on its net (nowhere to
connect → NC). Relabel to "dropped — lone pin on its net (→ NC):".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/tui/screen_dashboard.cpp

@@ -259,7 +259,7 @@ Component Tui::BuildDashboardScreen() {
       // spot a false positive.
       if (orph_dropped > 0) {
           lines.push_back(hbox({
-              text("        dropped detail:") | dim,
+              text("        dropped — lone pin on its net (→ NC):") | dim,
           }));
           for (auto &dkv : dropped_by_module) {
               std::sort(dkv.second.begin(), dkv.second.end(), NaturalLess);