feat: an app icon in colour and symbolic form, validated at build time

Both are drawn to the GNOME HIG: a 128px instrument face that leaves its
margins, carries one 4px front profile for depth and no gradients or shadows,
and a 16px symbolic reduction of the same metaphor in fills only. The colour
icon strokes its sine because it is never recoloured; the symbolic one cannot.

Every file in the manifest now preprocesses through xmllint, because a
malformed SVG is the one icon failure no runtime check can see: librsvg refuses
the file while the name still resolves, so the icon draws nothing and
has_icon() says it is fine.

src/icons/udpscope.gresource.xml

@@ -2,16 +2,26 @@
 <!-- SPDX-License-Identifier: MIT -->
 <!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
 <!--
-  The four icons this app had to draw for itself, compiled into the binary so there is
-  still exactly one file to run. `gtk_icon_theme_add_resource_path()` wants the tree an
-  icon theme has, so the prefix is followed by `<size>/<context>`; `scalable` is the one
-  that means "an SVG, use it at any size".
+  The icons this app had to draw for itself, compiled into the binary so there is still
+  exactly one file to run. `gtk_icon_theme_add_resource_path()` wants the tree an icon theme
+  has, so the prefix is followed by `<size>/<context>`; `scalable` is the one that means
+  "an SVG, use it at any size", and `apps` is where an application icon belongs.
+
+  Every file carries `preprocess="xml-stripblanks"`, which makes glib-compile-resources pipe
+  it through xmllint. That is the only thing that catches a malformed SVG: librsvg refuses
+  such a file outright, but the icon *name* still resolves, so `gtk_icon_theme_has_icon()`
+  returns true and the icon simply draws nothing. No runtime GTK call can tell the two apart
+  (`lookup_icon()` never returns NULL and always reports the size it was asked for), so the
+  check belongs here, where a double hyphen inside a comment fails the build with a file,
+  line and column instead.
 -->
 <gresources>
   <gresource prefix="/org/udpscope/icons">
-    <file>scalable/actions/udpscope-trigger-symbolic.svg</file>
-    <file>scalable/actions/udpscope-cursors-symbolic.svg</file>
-    <file>scalable/actions/udpscope-measure-symbolic.svg</file>
-    <file>scalable/actions/udpscope-math-symbolic.svg</file>
+    <file preprocess="xml-stripblanks">scalable/actions/udpscope-trigger-symbolic.svg</file>
+    <file preprocess="xml-stripblanks">scalable/actions/udpscope-cursors-symbolic.svg</file>
+    <file preprocess="xml-stripblanks">scalable/actions/udpscope-measure-symbolic.svg</file>
+    <file preprocess="xml-stripblanks">scalable/actions/udpscope-math-symbolic.svg</file>
+    <file preprocess="xml-stripblanks">scalable/apps/org.udpscope.app.svg</file>
+    <file preprocess="xml-stripblanks">scalable/apps/org.udpscope.app-symbolic.svg</file>
   </gresource>
 </gresources>