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/help_window.hpp

@@ -18,20 +18,26 @@ void show_about(GtkWindow* parent, const std::string& session_path);
 /// so the dialog is asserted to *show* the app icon rather than merely to open.
 const char* about_icon_name();
 
-/// The paged help window. Modal on nothing — it is a reference, so it stays usable while
-/// the scope runs. Already open: presented.
+/// Opens the installed Mallard manual in the desktop help viewer — Yelp, through the
+/// `help:` URI the freedesktop Help System Specification defines — rather than drawing a
+/// window of our own. `parent` is the window the launch is attributed to (and the one an
+/// error is reported on); the viewer is a separate process, so nothing here can be closed
+/// with `close_help_windows()`.
 void show_help(GtkWindow* parent);
 
-/// What UDPSCOPE_PROBE_HELP_MS reports. `parsed` counts pages whose markup
-/// pango_parse_markup() accepted and whose body is non-empty; it must equal `pages`.
+/// What UDPSCOPE_PROBE_HELP_MS reports about the manual *in the tree*. `topics` is the
+/// number of `<id>.page` files other than `index.page`, and `index` says whether
+/// `index.page` is present: the manual is now a set of files on disk (installed by the
+/// `install()` rule, and read by Yelp), not a GResource in the binary, so the self-check
+/// asserts the set is *there* and must be read as a file-system finding — `topics=0
+/// index=0` means the manual is missing from `UDPSCOPE_HELP_DIR`.
 struct HelpReport {
-  int pages = 0;
-  int parsed = 0;
-  std::string shortest_title;
-  std::size_t shortest_bytes = 0;
+  int topics = 0;
+  bool index = false;
+  int figures = 0;
 };
 
-/// Loads and validates every page without building a window. Safe to call headless.
+/// Scans the manual's directory without opening a viewer. Safe to call headless.
 HelpReport help_self_check();
 
 /// The shortcut overview: the application accelerators and the plot's own interactions,
@@ -39,11 +45,11 @@ HelpReport help_self_check();
 /// deprecated as of GTK 4.18 and this project builds warning-free.
 void show_shortcuts(GtkWindow* parent);
 
-/// Closes whichever of the three windows are open. One of them is a GtkWindow -- the help
-/// window, closed with gtk_window_close(), which emits close-request so its handler runs. The
-/// other two are AdwDialogs, which are not GtkWindows: they close with adw_dialog_close(),
-/// which emits ::closed. Either way the file-scope pointer is nulled by the handler, which is
-/// what prevents a second open presenting a dead widget. Used by the probe.
+/// Closes whichever of the two dialogs are open. Both are `AdwDialog`s, which are not
+/// `GtkWindow`s: they close with `adw_dialog_close()`, which emits `::closed`, and the
+/// file-scope pointer is nulled by the handler that prevents a second open presenting a dead
+/// widget. There is no help window left to close -- the manual opens in a separate viewer
+/// process. Used by the probe.
 void close_help_windows();
 
 }  // namespace udpscope