ui: the signals panel's frame is a .ui file

src/ui/signals-panel.ui holds the panel's shape: the box and its margins, the heading,
the hint that says what a signal is for, and the list. The rows stay in code -- one per
signal, carrying that signal's switch, colour, calibration and drop targets, rebuilt
whenever the stream's signals change -- so the file holds the frame, which is the shape
the plan gives this slice.

The list's width-only size request moved with it, comment and all: a height floor there
made the list occupy 120 px whatever it held, which hid the compact rows' height behind
it.

Read back by a new dump line, `signals panel: labels=2 list=1`, where a lost id reads as
`labels=1` or `list=0` instead of a panel quietly missing its hint; `panels: … row_h=26`
still reads the list's geometry and the height a row takes. check_ui is at 10 ok.

src/ui/signals-panel.ui

@@ -0,0 +1,43 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- SPDX-License-Identifier: MIT -->
+<!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
+<!--
+  The signals panel: a heading, the hint that says what a signal is for, and the list they go in.
+
+  The rows are not here. There is one per signal in the stream, it carries the switch that decides whether
+  that signal is acquired, its colour, its calibration and its drop targets, and the whole list is rebuilt
+  whenever the stream's signals change — so the file holds the frame and the code holds the rows.
+
+  The list's size request is **width only**, deliberately. A height floor here made the list occupy 120 px
+  whatever it held, which hid the compact rows' height behind it and wasted the space on an empty stream;
+  the scroller this panel sits in already caps how tall it can grow.
+-->
+<interface>
+  <object class="GtkBox" id="signal_panel">
+    <property name="orientation">vertical</property>
+    <property name="spacing">6</property>
+    <property name="margin-top">8</property>
+    <property name="margin-bottom">8</property>
+    <property name="margin-start">8</property>
+    <property name="margin-end">8</property>
+    <child>
+      <object class="GtkLabel" id="signals_heading">
+        <property name="label" translatable="yes">Signals</property>
+        <property name="halign">start</property>
+      </object>
+    </child>
+    <child>
+      <object class="GtkLabel" id="signals_hint">
+        <property name="label" translatable="yes">drag a signal onto a plot</property>
+        <property name="halign">start</property>
+        <property name="css-classes">dim-label</property>
+      </object>
+    </child>
+    <child>
+      <object class="GtkListBox" id="signal_list">
+        <property name="selection-mode">none</property>
+        <property name="width-request">260</property>
+      </object>
+    </child>
+  </object>
+</interface>

src/ui/udpscope-ui.gresource.xml

@@ -18,5 +18,6 @@
     <file preprocess="xml-stripblanks">cursors-panel.ui</file>
     <file preprocess="xml-stripblanks">measures-panel.ui</file>
     <file preprocess="xml-stripblanks">trigger-panel.ui</file>
+    <file preprocess="xml-stripblanks">signals-panel.ui</file>
   </gresource>
 </gresources>