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

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+<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>.

src/help/02-source.txt

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+<b>Choosing a source</b>
+
+A source is a host and a port, optionally reached through a multicast group on a named
+interface. Pass it on the command line, or let udpscope ask.
+
+<b>How to reach it</b>
+
+Run with <tt>--server</tt> and the app connects straight away. Run <i>without</i>
+<tt>--server</tt> and a chooser appears first: the sessions udpscope has stored, newest
+first, plus a <b>New host…</b> form for one it has not seen. The chooser runs before any
+buffer is allocated and before any socket is opened, so declining it costs nothing.
+
+<b>Worth knowing</b>
+
+• The list is the sessions that <i>named a signal</i>. A session whose streamer was down
+  stores empty slot names and is filtered out, which is what keeps the list the sources you
+  actually configured rather than every port you have ever typed.
+• A stored row reconnects the way it was reached. The multicast group and the interface are
+  written inside the session file, because the file's name (<tt>host_port.toml</tt>) cannot
+  carry them.
+• <tt>--multicast</tt> still needs <tt>--server</tt>: the CONFIG message is fetched from the
+  server, not from the group.
+• The <tt>×</tt> beside a row deletes that session file. Nothing else is removed.
+
+<b>Example</b>
+
+<tt>udpscope</tt> with two stored sessions lists
+<tt>127.0.0.1:8000  plots=1 signals=3</tt> and
+<tt>10.0.0.5:8001  plots=2 signals=4 multicast=239.1.2.3 interface=10.0.0.9</tt>.
+Activating the first reports <tt>[sources] picked 127.0.0.1:8000</tt> and continues into
+the scope with that session's layout already loaded.

src/help/03-signals.txt

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+<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.

src/help/04-layout.txt

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+<b>Rows, layout and drag &amp; drop</b>
+
+The figure is a stack of rows, 1 to 8 of them, each holding up to 4 traces. Rows share one
+time axis and one left gutter, so their y labels line up and their windows cannot drift apart.
+
+<b>How to reach it</b>
+
+Drag a signal's chip out of the side panel (<tt>s</tt>) and drop it on a row to add a trace.
+Press <tt>l</tt>, or pick <b>Layout editor</b> from the overflow menu, to add and remove rows:
+a <tt>×</tt> appears at the top left of each row and a <tt>+</tt> on each boundary between
+rows, including above the first and below the last. The editor's own button in the top-right
+corner leaves it.
+
+<b>Worth knowing</b>
+
+• A drop <i>adds</i> a trace to the row's next free slot; it does not replace one. With four
+  traces already on a row the drop is refused.
+• Inserting a row moves the rows below it <i>with their contents</i>. Inserting above row 1
+  puts what row 1 plotted onto row 2 and leaves the new row empty — the rows are not
+  renumbered under you.
+• A trace is remembered by the signal's <i>name</i>, not by its channel number, because a new
+  CONFIG or an enable renumbers the channels and an index would silently re-point the trace.
+• Adding or removing a row rebuilds the figure, which is cheap, but it also re-deals every
+  trace. A frozen trigger event survives the rebuild: the pinned window is restated rather
+  than re-framed on the live edge.
+
+<b>Example</b>
+
+With three traces on row 1, pressing <tt>l</tt> shows 3 buttons (one <tt>×</tt>, two
+<tt>+</tt>). Clicking the <tt>+</tt> above row 1 gives 2 rows and 5 buttons, with the three
+traces now on row 2 — the layout probe reports exactly this as
+<tt>rows=2 buttons=5 traces=[0:-|1:sine,ramp,noise]</tt>.

src/help/udpscope-help.gresource.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  SPDX-License-Identifier: MIT
+
+  The help pages, compiled into the binary for the same reason the icons are: udpscope is
+  run straight out of the build tree and has no install step, so a search path would have
+  to be located at runtime and the failure mode of not locating it is a help window full
+  of missing pages.
+-->
+<gresources>
+  <gresource prefix="/org/udpscope/help">
+    <file>01-overview.txt</file>
+    <file>02-source.txt</file>
+    <file>03-signals.txt</file>
+    <file>04-layout.txt</file>
+  </gresource>
+</gresources>