ui: the events pane's frame is a .ui file

src/ui/events-pane.ui holds the pane: the GtkRevealer that slides it in, the header with
the way back to the live stream, the Compare and Accumulate actions, and the list inside
its scroller. The rows stay in code -- one per latched event, rebuilt whenever the cache
changes -- and their sensitivity stays with update_group_buttons(), which is why the file
only starts the two actions insensitive.

Two statements moved into the file along with the widgets, because they are about what the
pane is for rather than about a default: multiple selection, and activation on *double*
click. With single-click activation the plain click that builds a selection would replay
the event instead.

Read back as `events pane: compare=1 accumulate=1 list=1 revealer=1 nested=1`, where
`nested` is the list being inside the revealer -- the one thing a lost id cannot show. The
[events] probe is the other half and drives the pane end to end: recording, trigger, the
history button, a row click and the replay, then Compare and Accumulate, so the file's
buttons are read by being clicked. check_ui: 13 ok, 0 failed.

docs/claude/probe-readings.md

@@ -1202,3 +1202,15 @@ button), and the list the channel rows go in. `buttons=0` is a `.ui` that lost t
 cannot add a channel, with nothing else complaining -- and `title=` is the file's own text read back, so a
 `.ui` that lost the label shows as `title='-'`. `[math-probe]` is the other half of the reading: it drives
 the add and the editor through the same code a user does.
+
+## `events pane:`
+
+```
+[udpscope]   events pane: compare=1 accumulate=1 list=1 revealer=1 nested=1
+```
+
+The events pane's frame read from the window: the two group actions, the list, and the revealer the `.ui`
+builds, plus `nested` — the list being *inside* that revealer, which is the one thing a lost id cannot show
+(a list parented elsewhere reads 1 everywhere else). `[events]` is the other half, and it drives the pane end
+to end: recording off, recording on, the trigger, the history button that reveals it, a row click and the
+replay, then Compare and Accumulate — so the file's buttons are read by being used.