Signal-type popup, NC pin tagging, interactive viewer hygiene.

- Enter on a signal entry (net / explore) opens a modal popup to pick
  power / gnd / other. Recording is deduped: a sequence of toggles on
  the same signal collapses to a single `set-signal-type` line; no-op
  selections record nothing.
- Bare interactive commands (the ones that open a full-screen mode)
  are no longer recorded by `script-save`. Their inline forms still
  are. Mutating actions inside a screen record their own canonical
  line.
- Mentor importer treats signals whose name starts with `unconnected`
  as no-connect — the pin is kept on the part without a signal and
  tagged `ImportedUnconnected`.
- `drop_singleton_signals` runs at the end of `load`: any signal with
  exactly one pin is detached (singletons are NC by definition); the
  pin is tagged `DroppedSingleton`. Count is reported inline.
- `verify` gains a one-line orphan summary (imported NC / dropped
  singleton totals). Pins materialised by `FillIdentityNCs` are
  excluded via a `pin_map` filter — they are bridged to a real signal
  on the peer module and are not real NCs at system level.
- NcOrigin tag is serialized in save snapshots as an optional 4th
  field on N records (backward-compatible).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/tui/tui.cpp

@@ -41,8 +41,10 @@ void Tui::Run() {
     auto search_screen  = BuildSearchScreen();
     auto connect_screen = BuildConnectScreen();
     auto settype_screen = BuildSettypeScreen();
-    auto explore_screen = BuildExploreScreen();
-    auto net_screen     = BuildNetScreen();
+    auto explore_screen = BuildExploreScreen() | Modal(BuildSignalTypeModal(),
+                                                       &sigtype_dialog_open);
+    auto net_screen     = BuildNetScreen()     | Modal(BuildSignalTypeModal(),
+                                                       &sigtype_dialog_open);
 
     auto tab = Container::Tab(
         {main_screen, search_screen, connect_screen, settype_screen, explore_screen,
@@ -50,6 +52,10 @@ void Tui::Run() {
         &screen_idx);
 
     auto root = CatchEvent(tab, [this](Event e) {
+        // The signal-type popup must own Escape / Tab while it's open so the
+        // outer switch doesn't yank us back to the main screen.
+        if (sigtype_dialog_open) return false;
+
         switch (screen_idx) {
         case 5:  // net
             if (e == Event::Escape) { screen_idx = 0; return true; }