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/source.page

@@ -0,0 +1,75 @@
+<page xmlns="http://projectmallard.org/1.0/"
+      type="topic" style="task"
+      id="source">
+
+  <info>
+    <link type="guide" xref="index#main"/>
+    <link type="seealso" xref="sessions"/>
+    <desc>How udpscope is pointed at a source: the command line, the startup chooser, and
+    opening an exported waveform instead of a live stream.</desc>
+    <credit type="author copyright">
+      <name>Martino Ferrari</name>
+    </credit>
+  </info>
+
+  <title>Choosing a source</title>
+
+  <p>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.</p>
+
+  <section id="reach">
+    <title>How to reach it</title>
+    <steps>
+      <item>
+        <p>Run with <cmd>--server</cmd> and the app connects straight away.</p>
+      </item>
+      <item>
+        <p>Run <em>without</em> <cmd>--server</cmd> and a chooser appears first: one row per
+        stored <em>configuration</em> (<code>host:port · name</code>, its note, and the
+        settings it carries), newest first, plus a <gui>New host…</gui> form that takes the
+        address <em>and</em> the configuration it belongs to — its name and note, the
+        <gui>buffer size</gui>, and whether time is relative or absolute. A row's pencil
+        button opens the same form on that configuration, pre-filled, and changes to its name
+        or address move it (the old file is deleted).</p>
+      </item>
+      <item>
+        <p>The second page, <gui>Waveforms</gui>, opens an <em>exported</em> run instead of a
+        live source: the files opened before (newest first; one that has since been deleted is
+        marked <em>missing</em>) and an <gui>Open…</gui> button for a new one. The same file
+        can be opened straight from the command line with <cmd>--open FILE</cmd>. A waveform
+        is a source that never arrives — no socket, no acquisition, no starvation notice — and
+        every instrument works on it: cursors, measurements, math channels and another export.
+        Only <file>.npy</file> files are read.</p>
+      </item>
+    </steps>
+
+    <p>The chooser runs before any buffer is allocated and before any socket is opened, so
+    declining it costs nothing.</p>
+  </section>
+
+  <section id="knowing">
+    <title>Worth knowing</title>
+    <list>
+      <item><p>The list is the sessions that <em>named a signal</em>. 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.</p></item>
+      <item><p>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
+      (<file>host_port.toml</file>) cannot carry them.</p></item>
+      <item><p><cmd>--multicast</cmd> still needs <cmd>--server</cmd>: the CONFIG message is
+      fetched from the server, not from the group.</p></item>
+      <item><p>The <gui>×</gui> beside a row deletes that session file. Nothing else is
+      removed.</p></item>
+    </list>
+  </section>
+
+  <section id="example">
+    <title>Example</title>
+    <p><cmd>udpscope</cmd> with two stored sessions lists
+    <code>127.0.0.1:8000  plots=1 signals=3</code> and
+    <code>10.0.0.5:8001  plots=2 signals=4 multicast=239.1.2.3 interface=10.0.0.9</code>.
+    Activating the first reports <code>[sources] picked 127.0.0.1:8000</code> and continues
+    into the scope with that session's layout already loaded.</p>
+  </section>
+
+</page>