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/01-overview.txt

@@ -0,0 +1,26 @@
+<b>What udpscope is</b>
+
+udpscope plots a live UDPStreamer source. One window holds up to 8 rows of plots, each
+with up to 4 traces; every row shares one time axis, so the whole figure scrolls together.
+Acquisition runs on its own thread and writes a ring buffer sized from --buffer-mb; the
+drawing thread reads whatever that ring holds when a frame starts.
+
+<b>How to reach it</b>
+
+Start the app with a source (<tt>udpscope --server 127.0.0.1 --port 8000</tt>), or start it
+with no <tt>--server</tt> and pick one from the chooser. The signals arrive disabled; turn
+one on in the side panel (<tt>s</tt>) and drag it onto a row.
+
+<b>Worth knowing</b>
+
+• Nothing is acquired until a signal is enabled. Enabling is what starts acquisition, so a
+  freshly opened scope with an idle side panel is working correctly.
+• The stream carries its own clock. Every time on screen is a stream timestamp, never the
+  local wall clock, which is why a stalled source freezes the view instead of scrolling it.
+• The window remembers itself. Rows, traces, the time window, calibration, math channels
+  and measurements are written to a session file per host and port, and reloaded next time.
+
+<b>Example</b>
+
+A 3-signal source at 10 kS/s with a 200 MB buffer holds 1677 s of history. Enable all three,
+drop them on row 1, and the status bar reads <tt>● live · 10.0 kS/s · 2 s window</tt>.