ui: one channel list, and the math pane is gone

`rebuild_signal_list()` is handed `model.signals()`, and `append_math_signals_locked()` already
appends one synthetic entry per math channel -- "they go through the ordinary signal path on
purpose: a derived channel is a channel" -- so **every math channel was already a row of the
signal list**, with its tick, chip, name, unit and menu. The math pane listed the same channels a
second time, so this is mostly a deletion: `build_math_panel()`, `rebuild_math_pane()`,
`src/ui/math-panel.ui` and its resource entry, `App::math_list`, and the `math panel:` dump line.

What the one list gains:

- a non-selectable `MATH` divider before the first math row, carrying the `+` that creates a
  channel -- appended even when there are no math channels at all, because a `+` conditional on
  one existing could never create the first one;
- a math row's own menu: *Edit graph…*, *Default colour*, *Delete*. No Information and no
  Calibrate (calibration is applied at ingest, and a derived channel has no stream to describe),
  and no rename, because the node editor owns a channel's name and unit -- a second place to
  change them would be a second answer to the same question. An item dismisses the row's popover
  before opening or destroying anything from it;
- `App::math` mirrored from a math row's tick: `enable_locked()` mirrors it into the model, but
  `App::math` is the copy the session file is written from. The mirror sits *before* the
  `apply_math_channels()` in the `!on` path, which would otherwise push a stale `enabled = true`
  straight back into the model;
- a tick visible in a file view too: a derived channel is computed, and a file view has no
  acquisition to start, which is the other tick's question.

`rebuild_signal_list()` also publishes the list's own shape -- `rows`, `signals`, `math` -- and
the dump prints `divider=1` when `rows == signals + 1 + math`: the reading for a list that lost
its `+`.

Verified: `make test-ui` 14 files (was 15); `signals panel: labels=2 list=1 rows=9 signals=4
math=4 divider=1`; `[math-probe] 3 math channels`, every other line unchanged; no `math panel:`
line, and zero `Gtk-CRITICAL`.

src/ui/math-panel.ui

@@ -1,49 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<!-- SPDX-License-Identifier: MIT -->
-<!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
-<!--
-  The math panel: a heading row with the button that adds a channel, and the list they go in.
-
-  The rows are not here. A math channel's row carries its name, its unit, its expression, its colour and its
-  own edit and remove buttons, all of which follow the channel — so the file holds the frame and the code
-  holds the rows, as in the measurements and signals panels beside it.
-
-  The heading takes the slack instead of the button, which is what keeps the `+` at the row's end however
-  long the title or the panel is.
--->
-<interface>
-  <object class="GtkBox" id="math_panel">
-    <property name="orientation">vertical</property>
-    <property name="spacing">4</property>
-    <property name="margin-top">6</property>
-    <property name="margin-bottom">6</property>
-    <property name="margin-start">6</property>
-    <property name="margin-end">6</property>
-    <child>
-      <object class="GtkBox" id="math_head">
-        <property name="orientation">horizontal</property>
-        <property name="spacing">8</property>
-        <child>
-          <object class="GtkLabel" id="math_title">
-            <property name="label" translatable="yes">Math channels</property>
-            <property name="halign">start</property>
-            <property name="hexpand">1</property>
-          </object>
-        </child>
-        <child>
-          <object class="GtkButton" id="math_add">
-            <property name="icon-name">list-add-symbolic</property>
-            <property name="css-classes">flat</property>
-            <property name="tooltip-text" translatable="yes">add a math channel</property>
-          </object>
-        </child>
-      </object>
-    </child>
-    <child>
-      <object class="GtkBox" id="math_list">
-        <property name="orientation">vertical</property>
-        <property name="spacing">2</property>
-      </object>
-    </child>
-  </object>
-</interface>

src/ui/udpscope-ui.gresource.xml

@@ -18,7 +18,6 @@
     <file preprocess="xml-stripblanks">main-window.ui</file>
     <file preprocess="xml-stripblanks">cursors-panel.ui</file>
     <file preprocess="xml-stripblanks">measures-panel.ui</file>
-    <file preprocess="xml-stripblanks">math-panel.ui</file>
     <file preprocess="xml-stripblanks">math-editor.ui</file>
     <file preprocess="xml-stripblanks">events-pane.ui</file>
     <file preprocess="xml-stripblanks">header-bar.ui</file>