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/signals.page

@@ -0,0 +1,70 @@
+<page xmlns="http://projectmallard.org/1.0/"
+      type="topic" style="task"
+      id="signals">
+
+  <info>
+    <link type="guide" xref="index#main"/>
+    <link type="seealso" xref="layout"/>
+    <desc>How the source's CONFIG becomes channels, what enabling a signal starts, and how a
+    signal's scale and offset are calibrated.</desc>
+    <credit type="author copyright">
+      <name>Martino Ferrari</name>
+    </credit>
+  </info>
+
+  <title>Signals and calibration</title>
+
+  <p>The source's CONFIG message describes its signals: a name, a unit and a type. udpscope
+  turns each enabled signal into a <em>channel</em> — 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.</p>
+
+  <section id="reach">
+    <title>How to reach it</title>
+    <steps>
+      <item>
+        <p>Open the side panel (<key>s</key>), which lists every signal with a switch.</p>
+      </item>
+      <item>
+        <p>The <gui>Signal list</gui> item in the overflow menu opens the editor where the
+        unit and the calibration live.</p>
+      </item>
+    </steps>
+  </section>
+
+  <section id="knowing">
+    <title>Worth knowing</title>
+    <list>
+      <item><p>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.</p></item>
+      <item><p><gui>Calibrate</gui> in a signal's row menu opens the gain and offset it is
+      <em>currently</em> drawn through, so a trace already scaled by a gain of 2 shows 2 —
+      pressing <gui>Apply</gui> confirms it rather than resetting it. A signal nobody
+      calibrated shows the identity, 1 and 0.</p></item>
+      <item><p>Calibration is applied <em>at ingest</em>, 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.</p></item>
+      <item><p>The stream's first signal is usually its <em>timestamp</em>, 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.</p></item>
+      <item><p>A signal's colour follows the <em>signal</em>, 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.</p></item>
+      <item><p><em>Click a coloured chip to change that signal's colour.</em> A grey chip
+      names no trace, so it is not clickable. The colour is saved with the session;
+      <gui>Default colour</gui> in the signal's ⋯ menu gives the palette entry back.</p></item>
+    </list>
+  </section>
+
+  <section id="example">
+    <title>Example</title>
+    <p>A source declaring <code>time</code>, <code>sine</code>, <code>ramp</code> and
+    <code>noise</code> yields three plottable signals. Enable all three and the exit dump
+    reads <code>3 signals, 3 channels</code>; <code>time</code> is not among them.</p>
+  </section>
+
+</page>