help: default the contextual layer off, and carry the help into the chooser

The contextual help was on by default, so a first run met a window with a "?"
beside every surface it had not asked to have explained; `contextual_help` now
defaults to false and the layer is asked for from the Help menu, from the
chooser's new help menu, or with Ctrl+H.

The source chooser runs before `App` exists, which makes it the window a user
who does not know how to start is looking at, so it now carries the same three
help affordances the window's Help view does: Documentation (the manual is
another process and needs nothing of the app), Enable contextual help (a row
whose check *is* the setting, written to the file the app reads a moment
later), and Guided tour. The tour is drawn on the app's own window, so the
chooser can only record the wish: it sets `Config::tour_pending` -- the value
`main()` owns and hands to both `choose_source()` and `run_ui()` -- and
`on_activate()` starts it once the window is up and laid out, through its own
callback so the readings can tell a request from the first-run auto-start.
The rows are the window's own `menu_button()`/`menu_toggle()`; `install_css()`
therefore runs from `choose_source()` (it is idempotent now) so the popover
gets the `.udpscope-menu` rules before it is shown.

Opening the manual could also fail with a message that blamed the handler
whatever was missing. `help_installed_path()` now resolves `help:<id>` the way
the specification does and the failure alert says which of the two it is: with
the set installed it names the viewer, without it it names the identifier, the
expected installed path and the install command. `show_help()` gained an
optional reporter, which `UDPSCOPE_PROBE_HELP_MS` uses for a new reading --
`[help] launch attempted= done= ok= error='' installed=` -- since a successful
launch leaves no window of ours to observe.

Tested: clean --clean-first build, three warnings as documented; test-ui 13/13
and test-help pass; no Gtk-CRITICAL in the help, helpui and chooser runs.

src/ui_app.hpp

@@ -255,6 +255,18 @@ struct App {
   /// The first-run auto-start: set by the timer that fires it, and read by the probe and the dump.
   /// The decision itself is `settings.tour_seen`, which is the whole of "this has happened before".
   bool tour_auto_started = false;
+  /// The chooser's own "Guided tour" request (`Config::tour_pending`), recorded when its timer runs.
+  /// A separate flag from the auto-start because it is a *different* thing: the user asked for the
+  /// tour before the window existed, rather than never having seen it.
+  bool tour_requested = false;
+  /// The self-check's reading of the manual's launch (`UDPSCOPE_PROBE_HELP_MS`). The launch is
+  /// asynchronous and the viewer is another process, so the outcome arrives in a callback and the
+  /// probe prints it from a timer afterwards; `attempted` is set at the call, because a launch that
+  /// never called back is itself a reading.
+  bool dbg_help_launch_attempted = false;
+  bool dbg_help_launch_done = false;
+  bool dbg_help_launch_ok = false;
+  std::string dbg_help_launch_error;
   /// The help layer's own dump readings, captured on the tick while the window is alive (nothing
   /// here may be read in the dump -- see `docs/claude/verification.md`).
   std::string dbg_context_help;
@@ -1743,6 +1755,10 @@ int tour_step_count(const App& app);
 int tour_step_index(const App& app);
 /// The first-run auto-start's callback, scheduled only when `settings.tour_seen` is false.
 gboolean tour_autostart(gpointer user);
+/// The chooser's "Guided tour" request (`Config::tour_pending`): the same delayed start, but it
+/// sets `tour_requested` rather than `tour_auto_started`, so the probe and the dump can tell the two
+/// apart. Scheduled only when the config carries the request.
+gboolean tour_start_requested(gpointer user);
 /// Captures the layer's own readings and the bounds of the surfaces the help's figures label, while
 /// the window is alive -- the dump runs after it is destroyed and may not ask GTK anything.
 void capture_help_state(App& app);