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.
