udpscope: an oscilloscope view for a UDPStreamer source

GTK4 and libadwaita. It receives one UDP stream through the vendored
udpsclient, buffers it in a large ring, and draws it with the vendored
plotter: a trigger with a pre/post event window, a pause that freezes the
window, cursors, measurements, math channels in a node editor, an event cache,
named configurations, preferences, and an export of the visible window to
.npy/.csv. Waveform files can be opened and analysed in place.

The interface is two threads -- a producer that owns the socket and the GTK
main thread that owns every widget and the GL context -- sharing one model
under one mutex, with samples crossing through lock-free rings.

deps/plotter is a submodule pinned to the commit this tree builds against;
deps/udpsclient is vendored in place. docs/claude/ documents the rules that
cost a defect to learn, and SPECS.md is the requirement document.

src/icons/scalable/actions/udpscope-resize-rows-symbolic.svg

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- SPDX-License-Identifier: MIT -->
+<!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
+<!-- A boundary that can be dragged: a vertical double-headed arrow. It says *which* edge the handle
+     moves, which a plain grip does not. Every coordinate is a whole pixel of the 16px cell, every
+     subpath is a filled `path` wound the same way (a stroke would keep this colour on a dark theme),
+     and the two heads meet the shaft exactly, at x 5..11 and x 7..9. -->
+<svg height="16px" width="16px" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg">
+  <path fill="#2e3436" d="M7 4 H9 V12 H7 Z
+                          M8 1 L11 5 H5 Z
+                          M8 15 L5 11 H11 Z"/>
+</svg>