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/status.page

@@ -0,0 +1,72 @@
+<page xmlns="http://projectmallard.org/1.0/"
+      type="topic" style="task"
+      id="status">
+
+  <info>
+    <link type="guide" xref="index#output"/>
+    <desc>The status bar's two tiers, the activity badge, the fixed state words, and the ⓘ
+    detail popover.</desc>
+    <credit type="author copyright">
+      <name>Martino Ferrari</name>
+    </credit>
+  </info>
+
+  <title>Status bar and diagnostics</title>
+
+  <p>The status bar is two tiers. The bar itself is one glance line — the state, the rate and
+  the window — and everything else is behind the ⓘ button at its right-hand end.</p>
+
+  <section id="reach">
+    <title>How to reach it</title>
+    <p>The bar is always visible under the plot. Click <gui>ⓘ</gui> for the detail popover.</p>
+  </section>
+
+  <section id="knowing">
+    <title>Worth knowing</title>
+    <list>
+      <item><p>The badge is one glyph in four colours: green <em>alternating</em> lit and dim
+      means data is arriving, and a steady colour means it is not. A badge that never blinks is
+      not a working badge. Of the two steady colours, <em>grey</em> is no stream at all or a
+      pipeline you stopped (the tooltip says which), and <em>yellow</em> is a source that
+      answered but has sent no configuration.</p></item>
+      <item><p>The state field is one word from a fixed set: <code>connecting</code>,
+      <code>live</code>, <code>paused</code>, <code>armed</code>, <code>capturing</code>,
+      <code>event held</code>, <code>disarmed</code>. Two states add one field each: in trigger
+      mode, how long ago the last event was; while paused, how long you have been paused.</p></item>
+      <item><p><code>connecting</code> means there is no <em>signal list</em> yet — either no
+      session or a session with no configuration behind it. Every other word claims a signal
+      list, so until one arrives there is nothing for them to describe: nothing can be enabled
+      and no data is owed.</p></item>
+      <item><p>The paused figure is <em>wall</em> time, not stream time — the producer never
+      stops while paused, so it answers "how long have I been looking at this picture".</p></item>
+      <item><p>Nothing on the bar may change width with the stream, because the bar sits
+      directly above the plot and a wider label would relayout it. That is why the rate
+      collapses to one figure or a range rather than listing per channel, and why an elapsed
+      time steps to a coarser unit instead of growing digits.</p></item>
+      <item><p><code>gaps</code> counts packets the receiver knows it missed. It is the number
+      that matters: a non-zero count means samples are absent from the buffer, not merely
+      late.</p></item>
+      <item><p><code>reconnects</code> counts sessions re-established with the source. Over
+      unicast a climbing count usually means a second client is taking the source's single
+      session.</p></item>
+      <item><p>The starvation notice — <em>connected, but no data is arriving</em> — is that
+      same situation seen from the other side: the signal list is here, so something took the
+      stream away. A source that has sent <em>no</em> configuration gets a different notice,
+      <em>trying to connect</em>, because nothing was taken away — pointing the app at a port
+      where nothing answers is that case, and the usual cause is a streamer that is not running
+      or is serving another port.</p></item>
+      <item><p><code>stored</code> versus <code>wire</code> in the popover is the decimation.
+      They differ exactly when the window is too long to store at full rate.</p></item>
+    </list>
+  </section>
+
+  <section id="example">
+    <title>Example</title>
+    <p>The bar reads <code>● live · 10 kS/s · 2 s window</code>. Behind ⓘ:
+    <code>Stream  connected, configured / packets 717 · frames 716 · gaps 0 / 3 signals · 3
+    channels</code>, <code>Buffer  200.0 MB · 1677 s span · time x1 / stored 10 kS/s (wire
+    10 kS/s)</code>, <code>Plot 1  3 traces · 195000 samples</code>,
+    <code>Reconnects  0</code>.</p>
+  </section>
+
+</page>