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/help/06-navigate.txt

@@ -6,7 +6,9 @@ pointer is over, and because the rows share one time axis, an x change moves all
 <b>How to reach it</b>
 
 <b>Wheel</b> zooms both axes · <b>Shift+wheel</b> zooms x only · <b>Ctrl+wheel</b> zooms y only.
-<b>Drag</b> pans · <b>Ctrl+drag</b> is a box zoom on x · <b>Alt+drag</b> a box zoom on y.
+<b>Drag</b> pans · <b>Shift+drag</b> is a box zoom on both axes · <b>Ctrl+drag</b> on x only ·
+<b>Alt+drag</b> on values only.
+The area being chosen is framed while you drag it, so you can see what the release will apply.
 <b>+</b> and <b>-</b> zoom both axes; <b>Ctrl</b> with them restricts to x, <b>Alt</b> to y.
 <b>Arrow keys</b> pan, <b>f</b> fits, <b>Escape</b> cancels a gesture in progress.
 The full table is in the shortcut overview (<tt>Ctrl+/</tt>).

src/help/08-trigger.txt

@@ -15,7 +15,7 @@ share. <b>Arm</b> starts watching; <tt>a</tt> arms and disarms from the keyboard
 <b>Worth knowing</b>
 
 • <b>Every triggered acquisition is kept.</b> Latches are written to the event cache
-  (<tt>~/.cache/udpscope/&lt;source&gt;/events/</tt>) as a <tt>.npy</tt> of the event, a
+  (<tt>~/.cache/udpscope/events/&lt;source&gt;/</tt>) as a <tt>.npy</tt> of the event, a
   <tt>.txt</tt> of what it was, and a <tt>.bmp</tt> picture drawn from the samples. The
   <b>Events</b> pane on the left -- opened from the toolbar's event-history button, which appears
   while recording is enabled, and collapsed until you ask for it -- lists the last 32 newest first