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_probe.cpp

@@ -4565,11 +4565,11 @@ void helpui_report_geometry(App& a, const char* what) {
   graphene_rect_t card = GRAPHENE_RECT_INIT(0, 0, 0, 0);
   GtkWidget* t = tour_target(a);
   const bool have_spot =
-      a.tour_spot != nullptr && gtk_widget_compute_bounds(a.tour_spot, a.window, &spot);
+      a.tour.spot != nullptr && gtk_widget_compute_bounds(a.tour.spot, a.window, &spot);
   const bool have_target = t != nullptr && gtk_widget_is_visible(t) &&
                            gtk_widget_compute_bounds(t, a.window, &target);
   const bool have_card =
-      a.tour_card != nullptr && gtk_widget_compute_bounds(a.tour_card, a.window, &card);
+      a.tour.card != nullptr && gtk_widget_compute_bounds(a.tour.card, a.window, &card);
   const auto contains = [](const graphene_rect_t& outer, const graphene_rect_t& inner) {
     return outer.origin.x <= inner.origin.x + 0.5 && outer.origin.y <= inner.origin.y + 0.5 &&
            outer.origin.x + outer.size.width >= inner.origin.x + inner.size.width - 0.5 &&
@@ -4607,7 +4607,11 @@ gboolean helpui_probe_step(gpointer user) {
     if (tour_active(*a)) tour_dismiss(*a);
     std::string ids;
     const int hints = context_help_count(*a, &ids);
-    std::fprintf(stderr, "[helpui] contextual on: hints=%d ids={%s}\n", hints, ids.c_str());
+    int hint_w = 0;
+    int hint_h = 0;
+    context_help_size(*a, &hint_w, &hint_h);
+    std::fprintf(stderr, "[helpui] contextual on: hints=%d ids={%s} hint=%dx%d\n", hints,
+                 ids.c_str(), hint_w, hint_h);
     int total = 0;
     int clicks = 0;
     helpui_count_controllers(a, &total, &clicks);
@@ -4736,11 +4740,11 @@ gboolean helpui_probe_step(gpointer user) {
                  "[helpui] after Skip: visible=%d dims_hidden=%d spot_hidden=%d card_hidden=%d "
                  "controllers=%d (was %d) click_gestures=%d\n",
                  tour_active(*a) ? 1 : 0,
-                 hidden(a->tour_dim[0]) && hidden(a->tour_dim[1]) && hidden(a->tour_dim[2]) &&
-                         hidden(a->tour_dim[3])
+                 hidden(a->tour.dim[0]) && hidden(a->tour.dim[1]) && hidden(a->tour.dim[2]) &&
+                         hidden(a->tour.dim[3])
                      ? 1
                      : 0,
-                 hidden(a->tour_spot) ? 1 : 0, hidden(a->tour_card) ? 1 : 0, total,
+                 hidden(a->tour.spot) ? 1 : 0, hidden(a->tour.card) ? 1 : 0, total,
                  a->dbg_helpui_controllers, clicks);
     capture_help_state(*a);
     std::fprintf(stderr, "[helpui] layer: %s\n", a->dbg_context_help.c_str());