ui: the math editor's window, identity row and canvas are a .ui file

The second half of this plan row, and the half of that dialog which is template.
src/ui/math-editor.ui holds the window (its size and modality), the box around it,
the identity row -- the name and the unit -- and the body with the canvas in it.

What stays in code is the drawing: the canvas is a GtkDrawingArea the file *declares*
and the code draws, with the gestures that make it a graph editor, and the parameter
form follows which node is selected. The palette and the buttons go next.

The builder is held on `App` and cleared on the *next* opening rather than with the
window: the window belongs to the builder's object list, so freeing the builder under a
live window frees the window with it. That is the source form's pattern, and the probe's
close-then-reopen path is what exercises it -- `reopened: dialog=ok, nodes=2`.

The identity row is read back too, since a `.ui` can hold the widget but not the value:
`dialog build: ... name='math3' unit='' ...`, the entries having been filled from the
channel being edited. check_ui is at 12 ok, 0 failed.

docs/superpowers/plans/2026-10-11-ui-files-and-gtk4-practice.md

@@ -74,6 +74,11 @@ needs a probe that proves it *loads* and that its contents match whatever drives
 | `src/ui/ui_layout.cpp` (~20 kB) | the layout editor's overlay buttons, which are positioned by code | only the buttons are template |
 | `build_toolbar` (`src/ui.cpp`, ~212 lines) | the header bar's layout, with every button still wired in code | delicate: the `[udpscope] header:` probe reads its geometry |
 
+**Done since, one row per commit:** the source chooser's window (`0a64e9e`), the measurements panel
+(`d344361`, `07fcf61`), the trigger panel (`1ccc70c`), the signals panel (`c37fbdd`), the math panel
+(`90a17eb`) and the math editor's window, identity row and canvas. What is left of that row is the palette
+and the buttons around the canvas, which stay in code for the reason the canvas's *drawing* does.
+
 **Done since:** the sidebar is an `AdwOverlaySplitView` (`src/ui/main-window.ui`) rather than a paned's end
 child, so its width is its own 320 px and it overlays the plot instead of collapsing to zero — see
 `[sidebar-probe]`.