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/measurements.page

@@ -0,0 +1,97 @@
+<page xmlns="http://projectmallard.org/1.0/"
+      type="topic" style="task"
+      id="measurements">
+
+  <info>
+    <link type="guide" xref="index#analysis"/>
+    <link type="seealso" xref="cursors"/>
+    <desc>A measurement is a statistic of the visible window of one signal, with optional
+    on-plot marks and a per-measurement statistics line.</desc>
+    <credit type="author copyright">
+      <name>Martino Ferrari</name>
+    </credit>
+  </info>
+
+  <title>Measurements</title>
+
+  <p>A measurement is a statistic of the <em>visible window</em> of one signal, recomputed as
+  that window moves. The pane lists them with their values; each can also draw its result on
+  the plot.</p>
+
+  <section id="reach">
+    <title>How to reach it</title>
+    <steps>
+      <item>
+        <p>Press the measurements button in the toolbar, or <key>m</key>.</p>
+      </item>
+      <item>
+        <p>The pane's <gui>+</gui> adds one; clicking a row opens the dialog where the signal,
+        the type, the threshold share and the on-plot marks are set, with <gui>Save</gui> and
+        <gui>Delete</gui>.</p>
+      </item>
+    </steps>
+  </section>
+
+  <section id="knowing">
+    <title>Worth knowing</title>
+    <list>
+      <item><p>The types are average, top, bottom, min, max, peak to peak, amplitude, pulse
+      duration, period and frequency. <em>top</em> and <em>bottom</em> are the means of the
+      upper and lower halves, which is why a unit sine reads 0.637 (2/π) rather than 1.</p></item>
+      <item><p>The on-plot marks come in two kinds and the mix is the reading: a <em>level</em>
+      mark is a ruler at a value, a <em>time</em> mark is a marker at a position. An average
+      draws one level mark; a max draws a level mark <em>and</em> a time mark at the first
+      sample that reached it.</p></item>
+      <item><p>Marks cannot be dragged. A measurement mark says what the measurement measured,
+      so it is created unselectable — as is the trigger marker.</p></item>
+      <item><p>A pass is throttled to a cadence rather than run every frame, so a ruler over a
+      live view is <em>stepped</em>. The number and the ruler come from the same pass, so they
+      always agree.</p></item>
+      <item><p>An invalid pass keeps its <em>level</em> marks and drops its <em>time</em>
+      marks. A level is a property of the signal and outlives the window; a position inside
+      the window does not. That is what stops the rulers blinking off at a mode change.</p></item>
+      <item><p>A crossing is counted with <em>hysteresis</em>, not on a bare level: the
+      thresholds are a tenth of the window's peak to peak either side of the share, and an
+      edge is counted only once the signal has passed the far one. Period, frequency and pulse
+      duration all depend on it. The mid level is exactly where a waveform moves fastest, so
+      any interferer faster than the fundamental recrosses a single level several times per
+      real crossing — a 5 Hz sine with a 500 Hz tone at a tenth of the amplitude reads 39 Hz
+      without the band and 5 Hz with it. The band holds to an interferer a third of the
+      waveform's own size.</p></item>
+      <item><p>The band is clamped to stay inside the data, which is what keeps a threshold set
+      near an extreme usable: a fixed band there would sit above every sample and the
+      measurement would read nothing. A share near an extreme still leaves the band little
+      room, so it rejects less — which is one reason period and frequency offer no share to
+      set.</p></item>
+      <item><p>A window with too few crossings reads <code>invalid</code> / <code>--</code>
+      rather than a plausible invented number. Frequency over a window holding one edge is the
+      usual case.</p></item>
+      <item><p><gui>frequency</gui> is reported in Hz and <gui>period</gui>/<gui>pulse
+      duration</gui> in seconds; every other type carries the channel's own unit. A value is
+      written with a <em>metric prefix</em> so its digits sit in <code>1..999</code> —
+      <code>636.62 mV</code>, not <code>0.64 V</code> — including when the stream declares no
+      unit at all, which is always the case for a math channel: there the prefix goes tight
+      against the number (<code>636.62m</code>), since two decimals of an unprefixed small
+      value is <code>0.00</code>.</p></item>
+      <item><p>Cost is bounded by reading the summary pyramid the producer already maintains,
+      not the raw window. The values are still exact: a bucket's sum is a sum.</p></item>
+      <item><p><gui>Statistics</gui> adds a second line to every measurement showing
+      <code>n</code>, the mean, the standard deviation and the range. Each observation is one
+      <em>recompute</em> of the pane, so <code>n</code> counts windows measured, not samples —
+      a mean frequency over many windows is what that claims to be. <gui>Reset</gui> clears
+      every accumulator and starts fresh. A change of display window, a CONFIG or a
+      measurement edit also resets; a pause, a resume, a zoom, a pan and the toggle itself do
+      not, so turning the line off to read the plot and back on again keeps the history.</p></item>
+    </list>
+  </section>
+
+  <section id="example">
+    <title>Example</title>
+    <p>On a 1 Hz ±1 sine: <code>average=0</code>, <code>top=0.636620</code>,
+    <code>bottom=-0.636620</code>, <code>max=1</code>, <code>min=-1</code>,
+    <code>peak to peak=2</code>, <code>amplitude=1</code>, <code>pulse duration=0.5 s</code>
+    (duty 0.5), <code>period=1 s</code>, <code>frequency=1 Hz</code>. Over a 2 M-sample window
+    those two passes read about 0.1% of the samples.</p>
+  </section>
+
+</page>