Clear the open minors: lost marking and overrun logging, form teardown guard, line breaks and ERANGE in config, probe strength

The E-1608 source marks every restart as lost and logs each overrun; its form stops touching
siblings after the chooser destroys them; the link skips command id 0, retries a full-buffer
discover send and ignores a zero-length scan read. Session files escape \n and \r; get_int and
get_double reject out-of-range values; the UDPS port spin reports changes; a skipped source panel
says why on stderr. The E-1608 probe now checks the signal names and the reconnect itself.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ui_source.cpp

@@ -6,6 +6,8 @@
 
 #include <adwaita.h>
 
+#include <cstdio>
+
 namespace udpscope {
 namespace ui_impl {
 
@@ -14,7 +16,11 @@ void build_source_panels(App& app) {
   SourceKind* kind = find_kind(app.source_kind);
   if (kind == nullptr) return;
   for (const SourcePanel& p : kind->make_panels(*app.acq->source())) {
-    if (p.widget == nullptr || p.id.empty()) continue;
+    if (p.widget == nullptr || p.id.empty()) {
+      std::fprintf(stderr, "[source] a '%s' panel was skipped: it has %s\n", app.source_kind.c_str(),
+                   p.widget == nullptr ? "no widget" : "an empty id");
+      continue;
+    }
     // Each page scrolls on its own, like Measure and Signals: panels differ in height.
     GtkWidget* scroller = gtk_scrolled_window_new();
     gtk_scrolled_window_set_policy(GTK_SCROLLED_WINDOW(scroller), GTK_POLICY_NEVER,