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/layout.page

@@ -0,0 +1,71 @@
+<page xmlns="http://projectmallard.org/1.0/"
+      type="topic" style="task"
+      id="layout">
+
+  <info>
+    <link type="guide" xref="index#main"/>
+    <link type="seealso" xref="signals"/>
+    <desc>How the figure's rows and traces are arranged, how to drop a signal onto a row, and
+    how the layout editor adds and removes rows.</desc>
+    <credit type="author copyright">
+      <name>Martino Ferrari</name>
+    </credit>
+  </info>
+
+  <title>Rows, layout and drag &amp; drop</title>
+
+  <p>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 pair of side margins, so their y labels line up, their data areas
+  start and end on the same two columns, and their windows cannot drift apart. The left
+  margin holds the y labels; the right one is empty, and is there so the newest tick's label
+  and the newest sample's ink have somewhere to land instead of sitting on the frame.</p>
+
+  <section id="reach">
+    <title>How to reach it</title>
+    <steps>
+      <item>
+        <p>Drag a signal's chip out of the side panel (<key>s</key>) and drop it on a row to
+        add a trace.</p>
+      </item>
+      <item>
+        <p>Press <key>l</key>, or pick <gui>Layout editor</gui> from the overflow menu, to add
+        and remove rows: a <gui>×</gui> appears at the top left of each row and a <gui>+</gui>
+        on each boundary between rows, including above the first and below the last.</p>
+      </item>
+      <item>
+        <p>The editor's own button in the top-right corner leaves it.</p>
+      </item>
+    </steps>
+  </section>
+
+  <section id="knowing">
+    <title>Worth knowing</title>
+    <list>
+      <item><p>A plot's own <gui>right-click menu</gui> (the same compact rows as the app's
+      menu) holds the y-axis scale, the two fits, and <gui>Remove signal</gui>, which swaps
+      the menu to that plot's traces plus <gui>All traces on this plot</gui> — with a
+      <gui>Back</gui> row to return. That is how a trace comes off a row without going through
+      the layout editor.</p></item>
+      <item><p>A drop <em>adds</em> 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.</p></item>
+      <item><p>Inserting a row moves the rows below it <em>with their contents</em>. 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.</p></item>
+      <item><p>A trace is remembered by the signal's <em>name</em>, not by its channel number,
+      because a new CONFIG or an enable renumbers the channels and an index would silently
+      re-point the trace.</p></item>
+      <item><p>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.</p></item>
+    </list>
+  </section>
+
+  <section id="example">
+    <title>Example</title>
+    <p>With three traces on row 1, pressing <key>l</key> shows 3 buttons (one <gui>×</gui>,
+    two <gui>+</gui>). Clicking the <gui>+</gui> above row 1 gives 2 rows and 5 buttons, with
+    the three traces now on row 2 — the layout probe reports exactly this as
+    <code>rows=2 buttons=5 traces=[0:-|1:sine,ramp,noise]</code>.</p>
+  </section>
+
+</page>