help: a tinier (i), hints beside their controls, and the tour starting in the chooser

The contextual help becomes the affordance the ask names: a plain (i)
(`dialog-information-symbolic`) at **16x16**, and one that sits beside the
control it explains rather than only in a pane's heading.

- The mark: `GtkMenuButton` ignores `min-width` on the widget and a size
  request, so the size is made in `install_css()` on all three nodes (outer
  `menubutton`, its `button` subnode, the glyph). Measured 16x16 against the
  default 34x34; the dump prints it as `hint=WxH`, measured with
  `gtk_widget_compute_bounds()` on the first *visible* hint.
- More sites: `events_record` (the "record events" switch in the trigger
  panel), `trigger_advanced` (the pre-trigger share row), `signals_config`
  (the signals pane's hint line, wrapped in a box so a hint can join it).
  Hints join as siblings, so none can cover a switch, pencil or dropdown.
- The source chooser gets its own layer -- `chooser_sources` (a new heading
  above the saved-source list) and `chooser_waveform` (the waveforms action
  row) -- built with the one `context_help_button()` both windows use and
  re-synced by its Help menu's toggle. The form's `AdwEntryRow`s cannot host a
  sibling hint, so none is invented for the host/port fields.
- The tour is one implementation over two windows: `ui_impl::TourView` plus a
  list of `TourStop`s; each window keeps its own table. A walk that passes its
  last stop calls `on_done`, and the chooser's sets `Config::tour_pending`, so
  a first run is one tour that crosses from the chooser into the app. The
  chooser's tour auto-starts from `settings.tour_seen` exactly as the window's
  does, and its "Guided tour" row now starts *it* instead of only requesting
  the window's.
- Probes: `UDPSCOPE_PROBE_CHOOSER_TOUR=1` / `UDPSCOPE_PROBE_CHOOSER_TOUR_MS`,
  the hint size in the `help layer:`/`[helpui] contextual on:` lines, and the
  chooser's hints in its `chooser ui:` line. Docs updated in
  invariants-gtk-and-chrome.md, probe-readings.md and verification.md.

src/ui_trigger.cpp

@@ -530,7 +530,10 @@ GtkWidget* build_trigger_panel(App& app) {
   row_for(ui, "level_row", app.trigger_level_spin);
 
   app.trigger_pre_spin = spin(0.0, 99.0, 1.0, 0, 50.0);
-  row_for(ui, "pre_row", app.trigger_pre_spin);
+  // The row is kept, not just filled: the contextual help's "trigger_advanced" hint joins it, beside
+  // the pre-trigger share -- the panel's finer settings, which is what the ask calls the trigger's
+  // advanced ones.
+  app.trigger_pre_row = row_for(ui, "pre_row", app.trigger_pre_spin);
 
   app.trigger_holdoff_spin = spin(0.0, 3600.0, 0.001, 4,
                                   app.model->config().trigger_holdoff_s);
@@ -564,6 +567,9 @@ GtkWidget* build_trigger_panel(App& app) {
     gtk_widget_set_halign(app.events_record_switch, GTK_ALIGN_END);
     gtk_widget_set_valign(app.events_record_switch, GTK_ALIGN_CENTER);
     gtk_box_append(GTK_BOX(record_row), app.events_record_switch);
+    // Where the contextual help's "events_record" hint joins -- beside the switch, never over it
+    // (see `ui_context_help.cpp`). Kept on the App because the layer's table names a field.
+    app.trigger_record_row = record_row;
   }
 
   app.arm_button = GTK_WIDGET(gtk_builder_get_object(ui, "trigger_arm"));