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/sessions.page

@@ -0,0 +1,93 @@
+<page xmlns="http://projectmallard.org/1.0/"
+      type="topic" style="task"
+      id="sessions">
+
+  <info>
+    <link type="guide" xref="index#output"/>
+    <link type="seealso" xref="source"/>
+    <desc>Per-source configuration files: where they live, what they persist, and how the
+    chooser and the Configurations dialog use them.</desc>
+    <credit type="author copyright">
+      <name>Martino Ferrari</name>
+    </credit>
+  </info>
+
+  <title>Sessions and settings</title>
+
+  <p>udpscope stores named sessions — <em>configurations</em> — per source, so reopening a
+  source gives you the scope you closed, and one source can hold several arrangements for
+  different experiments. There is no global preferences file: the session <em>is</em> the
+  settings.</p>
+
+  <section id="reach">
+    <title>How to reach it</title>
+    <p>Nothing to open — the file is written as you work. Its path is in the About dialog, and
+    the directory is <file>$XDG_CONFIG_HOME/udpscope</file>, or <file>~/.config/udpscope</file>
+    when that is unset. Each source has a directory <file>&lt;host&gt;_&lt;port&gt;/</file>
+    holding one <file>&lt;name&gt;.toml</file> per configuration and an <file>active</file>
+    file naming the one it is on.</p>
+  </section>
+
+  <section id="configs">
+    <title>Configurations</title>
+    <p><gui>Configurations…</gui> in the menu lists the running source's configurations: click
+    a row to open it, <gui>×</gui> to delete it, or type a name and a note and press
+    <gui>Save this session</gui> to keep the current arrangement under a new one. Opening a
+    configuration saves the current session first, so switching never loses what it switches
+    from.</p>
+
+    <p>The startup chooser (start with no <cmd>--server</cmd>) lists one row per configuration,
+    with its note and its settings, so two arrangements of one source are told apart before
+    they open.</p>
+
+    <p>A configuration also carries the settings that can only be decided when the buffers are
+    allocated: the <gui>buffer size</gui> (<cmd>--buffer-mb</cmd>), relative time, and the
+    packet capture's ring size. Opening a configuration from the chooser applies all three;
+    switching inside a running app applies the rest and says in the status line that these take
+    effect on the next start.</p>
+
+    <p>The chooser's form sets the first two for a new host, and its pencil button edits them
+    on an existing configuration — along with the name and the note. Renaming a configuration,
+    or editing the address it points at, moves it: the old file is removed rather than left
+    behind.</p>
+  </section>
+
+  <section id="knowing">
+    <title>Worth knowing</title>
+    <list>
+      <item><p>What a configuration persists: the time window, the rows and each row's slot
+      signals, the calibration, the trace colours, which signals were enabled, the math
+      channels with their graphs, the measurements pane — and the startup settings: buffer
+      size, relative time and the packet capture's ring size.</p></item>
+      <item><p>Signal identity is the <em>name</em> throughout, because a new CONFIG renumbers
+      the channels.</p></item>
+      <item><p>A file written by a run against a source whose streamer was down would name no
+      signals at all, so such a run does <em>not</em> write: a configured session is never
+      overwritten by a run that configured nothing. (The chooser hides such a file for the same
+      reason.)</p></item>
+      <item><p>The multicast group and the interface live <em>inside</em> the file, because its
+      name cannot carry them. A file from an older build has neither key and loads as unicast
+      on the default route.</p></item>
+      <item><p>The chooser lists only configurations that name at least one signal, which is
+      what keeps it the sessions you configured rather than every port you have ever tested
+      against.</p></item>
+      <item><p>Deleting a configuration is a supported way to start over; the chooser's
+      <gui>×</gui> and the dialog's do exactly that.</p></item>
+      <item><p>A session from before configurations existed is imported once as the
+      configuration <code>default</code>, and the old file is kept beside the directory as
+      <file>&lt;host&gt;_&lt;port&gt;.toml.imported</file>.</p></item>
+    </list>
+  </section>
+
+  <section id="example">
+    <title>Example</title>
+    <p>Connect to <code>127.0.0.1:8000</code>, drop three signals on row 1, set a 2 s window,
+    and save the session as <code>bench</code> with a note.
+    <file>~/.config/udpscope/127.0.0.1_8000/bench.toml</file> now holds
+    <code>window_s=2 comment="…"</code> with
+    <code>signals = ["sine", "ramp", "noise", ""]</code>, and the next run reports
+    <code>layout loaded from …/127.0.0.1_8000/bench.toml</code> with those signals already
+    acquired.</p>
+  </section>
+
+</page>