fix: five review findings on the tabbed-sidebar commit

Finding 2 — four builder ids had no null check. `side_stack`,
`side_switcher`, `measure_page` and `signals_page` were resolved at
lines 4709-4712, well past the skeleton null-guard. A typo in the `.ui`
would have been a silent segfault.  The four resolutions are moved to
the guard block and added to its condition, in the same form the guard
already uses.

Finding 3 — the 'editable focused: m refused' leg of keys_probe was
deleted instead of ported.  It is restored, adapted for the new
behaviour: 'm' now selects the Measure tab instead of toggling a pane,
so the refusal is observed as the side_tab name being unchanged across
the synthesised key, not as a toggle that did not flip.  The dump line
keeps its existing name and `refused=` field so probe-readings.md:601
still describes it.

Finding 4 — `exit_probe_step3` assigned `sidebar_shown` to both
`dbg_side_visible` and `dbg_side_column_after_hide`, and the panels:
dump line read the wrong one (dbg_side_visible) for its `(visible=)`
field.  Two readings that cannot disagree diagnose nothing.  Resolved
by making `panels:` read `dbg_side_column_after_hide` and removing the
redundant `dbg_side_visible` field.

Finding 5 — the three-line rationale comment above `reload_file_view`
in `apply_math_channels` was removed by the commit under review.
Nothing it said became untrue.  Restored verbatim from e10a92f.

Finding 6 — three comments the change made false:
- `on_activate`: 'the sidebar's scroller' replaced with 'the sidebar's
  tab switcher and two-page stack', matching the actual ui file.
- `update_side_visibility`: 'Hiding the scroller instead left an empty
  column where the panels should be' removed; the design it described
  no longer exists.
- `header-bar.ui`: 'two of them are toggles' updated to 'one of them
  is a toggle'; after signal_toggle's deletion the overflow menu holds
  one toggle (layout).

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

src/ui_probe.cpp

@@ -103,7 +103,6 @@ gboolean exit_probe_step3(gpointer user) {
   a->dbg_side_column_after_hide = sidebar_shown;
   a->dbg_switcher_visible =
       a->side_switcher != nullptr && gtk_widget_get_visible(a->side_switcher) ? 1 : 0;
-  a->dbg_side_visible = sidebar_shown;
   if (a->trigger_signal_drop != nullptr) {
     GListModel* model = gtk_drop_down_get_model(GTK_DROP_DOWN(a->trigger_signal_drop));
     a->dbg_trigger_items = model != nullptr ? static_cast<int>(g_list_model_get_n_items(model)) : 0;
@@ -4051,7 +4050,22 @@ gboolean keys_probe(gpointer user) {
   before_after("l: layout", a->layout_toggle, AppAction::LayoutEditor);
   before_after("s: side panel", a->sidebar_toggle, AppAction::SidePanel);
 
-  line += "\n";
+  // An accel must be refused while text is being typed. Focus the window entry, send 'm'
+  // through the real handler, and require the side tab not to change.
+  int refused = -1;
+  if (a->window_entry != nullptr) {
+    gtk_menu_button_popup(GTK_MENU_BUTTON(a->window_button));
+    gtk_widget_grab_focus(a->window_entry);
+    const std::string before_tab = a->side_tab;
+    const gboolean handled = on_app_key_pressed(nullptr, GDK_KEY_m, 0,
+                                                static_cast<GdkModifierType>(0), a);
+    const std::string after_tab = a->side_tab;
+    refused = (handled == FALSE && before_tab == after_tab) ? 1 : 0;
+    gtk_menu_button_popdown(GTK_MENU_BUTTON(a->window_button));
+  }
+  char tail[64];
+  std::snprintf(tail, sizeof(tail), " editable focused: 'm' refused=%d\n", refused);
+  line += tail;
 
   std::fprintf(stderr, "%s", line.c_str());
   std::fflush(stderr);