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_signals.cpp

@@ -594,6 +594,9 @@ GtkWidget* build_signal_panel(App& app) {
   app.signal_list = GTK_WIDGET(gtk_builder_get_object(ui, "signal_list"));
   // The heading row, where the contextual help's "?" joins (see `ui_context_help.cpp`).
   app.signals_head = GTK_WIDGET(gtk_builder_get_object(ui, "signals_head"));
+  // The hint line under it, which is where the "signals_config" hint joins: the heading already
+  // carries one, and the rule the layer follows is a hint beside the thing it explains.
+  app.signals_note = GTK_WIDGET(gtk_builder_get_object(ui, "signals_note"));
   app.signals_edit = GTK_WIDGET(gtk_builder_get_object(ui, "signals_edit"));
   if (app.signals_edit != nullptr) {
     g_signal_connect(app.signals_edit, "clicked", G_CALLBACK(on_pane_edit_clicked), &app);