feat: the manual is a Mallard set opened in Yelp, not a GResource in the binary

The help was 14 Pango-markup `.txt` bodies in a second GResource, drawn into a paged `GtkWindow`
built in `help_window.cpp`. That made F1 a viewer of ours with no search index, no cross-references
and no translation, and it kept the manual in the *binary* when every other desktop application's is
a document on disk. It is now a real Mallard set:

  help/C/org.udpscope.app/  index.page + 14 topic pages + figures/*.png

and `show_help()` hands `help:org.udpscope.app` to whatever the desktop registered for the `help:`
scheme -- Yelp on GNOME -- through a `GtkUriLauncher`, reporting a launch failure with a
`GtkAlertDialog` rather than a click that silently does nothing. The paged window's builder, the
page table and `help_self_check()`'s markup validation are gone with the resource.

The document identifier is the desktop file's own basename (its desktop-file id), derived in
CMakeLists from `data/org.udpscope.app.desktop.in`, so the same string is the `.desktop` name, the
GApplication id and the install directory at once. The set is installed by a new `install()` rule to
`<datadir>/help/C/org.udpscope.app/` -- where the freedesktop Help System Specification's `help:`
URI resolves `help:<id>` -- so the manual is read from a datadir by another process even though the
binary has no install step. `UDPSCOPE_HELP_NAME` and `UDPSCOPE_HELP_DIR` are compiled in from that
one place.

The 14 pages were migrated fact for fact, but restructured the Mallard way: a `<desc>` per page
(what a search result shows), `<steps>` for procedures, `<terms>` for option lists,
`<gui>`/`<key>`/`<keyseq>` for interface words and keys, `<note>` for the asides, and a topic graph
via `<link type="guide" xref="index#main"/>`. The figures are annotated screenshots produced by
`tools/make-help-shots.sh`, which drives the app headlessly against `tools/mock_udpstreamer.py` and
draws the boxes with ImageMagick; the side panel's box is measured from the app's own `side=NNNx`
dump reading, the header and status bands come from a documented table, and a crop whose standard
deviation is 0 fails the script.

`cmake --build build --target test-help` checks the set the way `test-ui` checks the `.ui` files:
`yelp-check`'s offline subcommands (links, media, orphans, ids) and a `yelp-build html` render.
`yelp-check validate` is deliberately absent -- it fetches the Mallard RelaxNG schema over the
network and fails offline for that reason, not for a fault in the pages.

The `[help]` probe reading changes with the resource it was written for. `parsed == pages` over
in-binary markup cannot exist now, so `help_self_check()` scans `UDPSCOPE_HELP_DIR` and the line
reads `topics=14 index=1 figures=3`; `topics=0 index=0` is the manual missing from the tree, which
is the finding the old count named.

Verified: clean build with the documented three warnings (the help bundle's `-Woverlength-strings`
is gone, so the baseline moves from four to three); test-ui 14/14; test-help clean (the four
subcommands and 15 pages rendered); a temp-prefix install lands 15 `.page` files and 3 figures at
`share/help/C/org.udpscope.app/`; the headless run reads `[help] topics=14 index=1 figures=3 dir=...`
with 0 Gtk-CRITICAL. Yelp's own window could not be exercised on this box -- it starts under Xvfb,
activates its portals and never maps, the same WebKit/portal limitation that stops a grab -- so its
rendering and its search index are unverified here; the content pipeline was (`yelp-build` renders
the whole set).

Docs: the manual's build/install/open story in `build-and-run.md` and README; the warning baseline
in `verification.md`/CLAUDE.md; the help bullets in `invariants-gtk-and-chrome.md`; the `[help]`
line in `probe-readings.md`.

help/C/org.udpscope.app/pause.page

@@ -0,0 +1,61 @@
+<page xmlns="http://projectmallard.org/1.0/"
+      type="topic" style="task"
+      id="pause">
+
+  <info>
+    <link type="guide" xref="index#viewing"/>
+    <link type="seealso" xref="trigger"/>
+    <desc>Pausing freezes the display without stopping the stream, and the status bar says how
+    long you have been paused.</desc>
+    <credit type="author copyright">
+      <name>Martino Ferrari</name>
+    </credit>
+  </info>
+
+  <title>Pause and play</title>
+
+  <p>Pause freezes the <em>display</em>, not the acquisition. The producer keeps filling the
+  ring at full rate; the view binds an immutable snapshot of the window that was on screen,
+  and the axis is pinned to it.</p>
+
+  <section id="reach">
+    <title>How to reach it</title>
+    <steps>
+      <item>
+        <p>Press the pause button in the toolbar, or <key>Space</key>.</p>
+      </item>
+      <item>
+        <p>Press it again to resume, which rebinds the live ring and puts the window back on
+        the newest data.</p>
+      </item>
+    </steps>
+  </section>
+
+  <section id="knowing">
+    <title>Worth knowing</title>
+    <list>
+      <item><p>Nothing is lost while paused. Because the producer never stops, resuming shows
+      current data with no hole in the history behind it.</p></item>
+      <item><p>The axis is pinned to the <em>snapshot's own stamps</em>, not to the window that
+      was requested. A window reaching back past the oldest retained sample yields a shorter
+      snapshot, and the pinned axis is that shorter span — so you see a narrower view rather
+      than an empty margin.</p></item>
+      <item><p>A paused view is pinned exactly as a held trigger event is: zoom in freely, but
+      a pan or a zoom out is clamped to the snapshot's data.</p></item>
+      <item><p>Pausing is the easiest way to measure a layout or read a cursor precisely,
+      because nothing under the pointer moves between frames.</p></item>
+      <item><p>The status bar says how long you have been paused — wall time, since the stream
+      itself never stopped. It is there so a frozen picture cannot be mistaken for a live one
+      that went quiet.</p></item>
+    </list>
+  </section>
+
+  <section id="example">
+    <title>Example</title>
+    <p>Pause with a 2 s window over a ring holding 1.4 s: the frozen axis reads
+    <code>[-1.40 .. 0.00]</code> rather than <code>[-2.00 .. 0.00]</code> with 0.6 s of blank.
+    The pause probe reports <code>ring_produced</code> still growing across the pause and
+    <code>event_size</code> non-zero exactly while paused.</p>
+  </section>
+
+</page>