feat: paged help window with the four Getting started pages

The pages are Pango markup in a GResource, bundled for the same reason the icons
are: udpscope runs straight out of the build tree with no install step, so a
runtime search path's failure mode would be a silently empty help window.
--c-name=udpscope_help keeps the two generated bundles from colliding.

Every page is validated with pango_parse_markup() at load and falls back to a
one-line stand-in rather than rendering raw markup or nothing. show_help() reuses
help_window.cpp's existing close-request handler, and closes via
gtk_window_close() so that handler runs and the pointer is nulled.

UDPSCOPE_PROBE_HELP_MS loads and validates every page, opens the help and About
windows, closes both and reports; parsed must equal pages.

src/help_window.hpp

@@ -3,6 +3,7 @@
 
 #include <gtk/gtk.h>
 
+#include <cstddef>
 #include <string>
 
 namespace udpscope {
@@ -11,4 +12,24 @@ namespace udpscope {
 /// Modal on `parent`. `session_path` is this run's own session file.
 void show_about(GtkWindow* parent, const std::string& session_path);
 
+/// The paged help window. Modal on nothing — it is a reference, so it stays usable while
+/// the scope runs. Already open: presented.
+void show_help(GtkWindow* parent);
+
+/// What UDPSCOPE_PROBE_HELP_MS reports. `parsed` counts pages whose markup
+/// pango_parse_markup() accepted and whose body is non-empty; it must equal `pages`.
+struct HelpReport {
+  int pages = 0;
+  int parsed = 0;
+  std::string shortest_title;
+  std::size_t shortest_bytes = 0;
+};
+
+/// Loads and validates every page without building a window. Safe to call headless.
+HelpReport help_self_check();
+
+/// Closes whichever of the three windows are open, via gtk_window_close() so the
+/// close-request handlers run and the pointers are nulled. Used by the probe.
+void close_help_windows();
+
 }  // namespace udpscope