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/03-signals.txt

@@ -0,0 +1,30 @@
+<b>Signals and calibration</b>
+
+The source's CONFIG message describes its signals: a name, a unit and a type. udpscope turns
+each enabled signal into a <i>channel</i> — a ring of values beside the shared time ring —
+and a channel is what a trace plots. Calibration is a scale and an offset applied at ingest.
+
+<b>How to reach it</b>
+
+The side panel (<tt>s</tt>) lists every signal with a switch. The <b>Signal list</b> item in
+the overflow menu opens the editor where the unit and the calibration live.
+
+<b>Worth knowing</b>
+
+• Signals start disabled, and enabling one is what starts acquiring it. A trigger on a signal
+  nobody acquires can never fire, so arming a trigger enables its signal for you.
+• Calibration is applied <i>at ingest</i>, so changing it re-plans the buffer and drops the
+  history. So does enabling or disabling a signal, and so does a new CONFIG. An empty plot
+  for a second after one of those is the ring refilling, not a fault.
+• The stream's first signal is usually its <i>timestamp</i>, and that one is never acquired.
+  It cannot be plotted, measured or used as a math source, and udpscope refuses to put it in
+  a row rather than binding a trace that would read nothing for ever.
+• A signal's colour follows the <i>signal</i>, not the slot, so the same signal on two rows
+  is one colour and the side panel's chip identifies it unambiguously. A signal on no row
+  gets a grey chip rather than none, so the panel's rows keep their height.
+
+<b>Example</b>
+
+A source declaring <tt>time</tt>, <tt>sine</tt>, <tt>ramp</tt> and <tt>noise</tt> yields three
+plottable signals. Enable all three and the exit dump reads <tt>3 signals, 3 channels</tt>;
+<tt>time</tt> is not among them.