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`.

docs/claude/build-and-run.md

@@ -52,6 +52,28 @@ here is needed to *build* the application. `blueprint-compiler` and `python3` ar
 time; without them the target is absent rather than silently passing. `tools/check_ui.py --lint` also
 prints the linter's advice, which is advice and deliberately not a gate.
 
+The manual has its own check, the same shape:
+
+```sh
+cmake --build build --target test-help
+```
+
+That runs `yelp-check`'s four offline subcommands over `help/C/<id>/` (links, media, orphans, ids)
+and renders the whole set with `yelp-build html` into a temp directory, so the manual is checked the
+way the `.ui` files are. **`yelp-check validate` is deliberately not used**: it fetches the Mallard
+RelaxNG schema over the network, which is absent on a build box, so it would fail for that reason and
+not for a fault in the pages. `yelp-check`/`yelp-build` are found at configure time; without them the
+target is absent rather than silently passing.
+
+The manual is a Mallard set under `help/C/<id>/`, installed by an `install()` rule to
+`<datadir>/help/C/<id>/` — where the freedesktop Help System Specification's `help:` URI resolves
+`help:<id>` — so it is *read from a datadir by another process* even though the binary itself has no
+install step. `F1` opens it in Yelp through that URI. The document identifier is the desktop file's
+basename (`data/org.udpscope.app.desktop.in` → `org.udpscope.app`), so it is the `.desktop` name, the
+GApplication id and the install directory at once. The figures are regenerated by
+`tools/make-help-shots.sh`, which drives the app headlessly against the mock and draws the annotation
+boxes with ImageMagick; Mallard is translatable with `itstool`, but only the `C` set is authored.
+
 Options: `--server`, `--port`, `--multicast`, `--interface`, `--buffer-mb` (per
 buffer, default 200), `--max-window`, `--holdoff`, `--recv-buffer`, `--plots` (the row
 count to *start* with, 1..`RowSet::kMaxRows`; the layout editor owns it afterwards),