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.

README.md

@@ -455,7 +455,7 @@ Trigger mode keeps the last **32 triggered acquisitions** in a cache, so an even
 time to look at is still there:
 
 ```
-~/.cache/udpscope/192.168.1.40-8000/events/
+~/.cache/udpscope/events/192.168.1.40-8000/
   20261009-123250-0001.npy   # the event, in the export format
   20261009-123250-0001.txt   # when, how long, how many signals, the rate, the edge
   20261009-123250-0001.bmp   # a picture of it, drawn from the samples
@@ -600,6 +600,28 @@ A session written before configurations existed is imported on first use as the
 named `default`: copied into the directory, remembered as active, and the old file renamed to
 `<host>_<port>.toml.imported` rather than deleted, so the import can be undone by hand.
 
+## Preferences
+
+`~/.config/udpscope/settings.toml`, written by **Preferences…** -- in the overflow menu or the source
+chooser -- and read at start-up. It holds the event cache's directory, how many events to keep (`0`
+keeps every one), whether relative time is on, and the interface's theme. Whether anything is cached at
+all is the trigger pane's own switch, and there is deliberately no second one here; the directory is
+the one place every source gets a folder inside, `<cache>/udpscope/events` by default. The cache
+directory and relative time are *start-up* settings -- the buffers are planned and the cache directory
+is opened before any source is known -- and the dialog says so; the theme applies as soon as it is
+chosen. A key that is absent means its default, so the file only mentions what changed, and a missing
+file is a first run rather than an error. The directory field shows the directory in use: the default
+is filled in rather than described, and typing it back leaves the setting unset, so it goes on
+following the cache directory instead of pinning one path for good. It can be typed, or picked with the folder
+button beside it, which writes what it found into the same field -- the field stays the one value, and
+there is nothing to keep in step.
+
+The same dialog is reachable from the source chooser, the one window that exists before a run is
+planned. From the app the cache directory and relative time take effect on the next start; from the
+chooser they reach the run about to start. A preference applied there also beats a source's stored
+configuration, exactly as `--relative-time` does: it is the more recent statement of intent, and it
+works in both directions, so turning relative time off in the dialog is not undone by the row you click
+afterwards.
 ## The session layout file
 
 A configuration's file holds the display