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`.

src/ui_probe.cpp

@@ -4447,7 +4447,10 @@ gboolean help_probe(gpointer user) {
   const std::string session =
       layout_path(a->model->config().server, a->model->config().port);
 
-  show_help(GTK_WINDOW(a->window));
+  // The manual is not opened here: it lives in another process (Yelp, through the `help:` URI)
+  // and neither it nor this box can draw a window for it, so there is nothing to open, read
+  // back or close. `help_self_check()` above is the assertion -- the set is on disk where the
+  // build says it is. The two AdwDialogs below are still built, read and closed for real.
   show_about(GTK_WINDOW(a->window), session);
   show_shortcuts(GTK_WINDOW(a->window));
   // Read while it is open: the icon is the point of the dialog carrying one, and
@@ -4464,11 +4467,11 @@ gboolean help_probe(gpointer user) {
   // works and is not documented, or is documented and does nothing.
   const UiShortcuts ui = ui_shortcut_rows("/org/udpscope/ui/shortcuts.ui");
   std::fprintf(stderr,
-               "[help] pages=%d parsed=%d shortest='%s' %zu B; shortcuts: app=%d plot=%d tables=%d "
+               "[help] topics=%d index=%d figures=%d dir=%s; shortcuts: app=%d plot=%d tables=%d "
                "ui=%d ui_accel=%d ui_icon=%d ui_bare=%d; "
                "about version=%s commit=%s built=%s settings=%s icon=%s; closed ok\n",
-               report.pages, report.parsed, report.shortest_title.c_str(),
-               report.shortest_bytes, accel_count, gesture_count, accel_count + gesture_count,
+               report.topics, report.index ? 1 : 0, report.figures, UDPSCOPE_HELP_DIR,
+               accel_count, gesture_count, accel_count + gesture_count,
                ui.rows, ui.with_accel, ui.with_icon, ui.bare, build_info::kVersion,
                build_info::kCommit, build_info::kTimestamp, config_dir().c_str(),
                about_icon.c_str());