ui: the math panel's frame is a .ui file

The first half of this plan row. src/ui/math-panel.ui holds the panel: the box and its
margins, the heading row with the title and the add button, and the list the channel rows
go in. The rows stay in code -- each is a channel with its name, unit, expression, colour
and its own edit and remove buttons -- so the file holds the frame, as in the measurements
and signals panels beside it.

Read back by a dump line that also reads the file's own text:
`math panel: head=2 buttons=1 list=1 title='Math channels'`, where buttons=0 is a lost
`+` and title='-' a lost label. `[math-probe]` still drives the add and opens the editor,
so the rows landing in the file's list is read as well as the frame.

The other half is the node editor's chrome, which is where least of that file is template:
the canvas is a GtkDrawingArea that is drawn rather than declared, and the gestures, the
parameter fields and the palette are code. That is its own change.

docs/claude/probe-readings.md

@@ -1190,3 +1190,15 @@ The signals panel's frame read from the window: the heading and the hint the `.u
 the rows go in. `labels=1` is a `.ui` that lost one of the two, `list=0` one that lost the list -- both of
 which otherwise show as a panel quietly missing a piece. `panels: … signals=WxH row_h=26` is the other half:
 the list's own geometry and the height a row really takes, which is what the compact-row CSS rule is for.
+
+## `math panel:`
+
+```
+[udpscope]   math panel: head=2 buttons=1 list=1 title='Math channels'
+```
+
+The math panel's frame read from the window: the head row the `.ui` gives it (the title and the add
+button), and the list the channel rows go in. `buttons=0` is a `.ui` that lost the `+` -- a panel that
+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.