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/layout_store.hpp

@@ -20,6 +20,9 @@ struct LayoutState {
   struct Row {
     /// One entry per slot; an empty string is an empty slot.
     std::vector<std::string> signals;
+    /// The row's share of the figure's height, 1 for equal rows. Part of the layout because a sizing a
+    /// user dragged is an arrangement, and an arrangement that is not saved is one that is lost.
+    double weight = 1.0;
   };
 
   double window_s = 0.0;  ///< 0 keeps the default window
@@ -126,6 +129,10 @@ std::string config_dir();
 /// lives (the packet captures; item 9's event cache). Created on demand by callers.
 std::string cache_dir();
 
+/// Creates `path` and every missing component, like `mkdir -p`. Public because the preferences are
+/// written under the config directory too.
+void mkdirs(const std::string& path);
+
 /// The *legacy* single-file path, `~/.config/udpscope/<host>_<port>.toml`, with the directory
 /// created. Still the import source for a file written before configurations existed; nothing
 /// writes here any more.