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`.

CLAUDE.md

@@ -38,7 +38,7 @@ The detail lives in `docs/claude/`. Read the files that touch the area you are e
 | File | What it covers |
 | --- | --- |
 | [build-and-run.md](docs/claude/build-and-run.md) | Configuring and building, the desktop-entry target, the command-line options, and the two vendored dependencies (`deps/plotter`, `deps/udpsclient`) including what was changed upstream and why. |
-| [verification.md](docs/claude/verification.md) | How to run the headless self-check, the **two baselines every run is read against** (exactly four build warnings; no `Gtk-CRITICAL`), and the environment variable that schedules each probe. |
+| [verification.md](docs/claude/verification.md) | How to run the headless self-check, the **two baselines every run is read against** (exactly three build warnings; no `Gtk-CRITICAL`), and the environment variable that schedules each probe. |
 | [probe-readings.md](docs/claude/probe-readings.md) | The per-line reference for the self-check's output: what every probe and dump line means, the value it must read, and the defect a wrong reading names. |
 
 ### How the application is put together
@@ -60,4 +60,4 @@ The detail lives in `docs/claude/`. Read the files that touch the area you are e
 | [invariants-measurements.md](docs/claude/invariants-measurements.md) | Why the pane is refreshed on a cadence, why neither pass may read its window raw, and how a crossing is counted with hysteresis. |
 | [invariants-math-channels.md](docs/claude/invariants-math-channels.md) | A math channel *is* a channel; the engine runs on the producer thread, so a regression there costs packets and not frames; the node editor's own rules. |
 | [invariants-session-and-export.md](docs/claude/invariants-session-and-export.md) | What a session file persists and in what order, when a save is refused, and when an export is collected. |
-| [invariants-gtk-and-chrome.md](docs/claude/invariants-gtk-and-chrome.md) | The GTK signal-signature and dialog-lifetime traps this app has already fallen into, the status bar's two tiers, the six bundled icons and their authoring rules, and the help pages. |
+| [invariants-gtk-and-chrome.md](docs/claude/invariants-gtk-and-chrome.md) | The GTK signal-signature and dialog-lifetime traps this app has already fallen into, the status bar's two tiers, the six bundled icons and their authoring rules, and how the Mallard manual is opened. |