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/claude/invariants-math-channels.md

@@ -39,7 +39,10 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   `math cost: S slots, N ns/slot (T timed), X ms total est` is the per-slot cost; see the
   math-cost invariant below for why it is sampled, why it is an upper bound, and why a figure
   anywhere in 78--101 ns/slot is one reading of the same thing.
-  The probe also asserts the new editor mechanics. Each line carries `(want ...)` and the reading
+  The probe also asserts the new editor mechanics. `dialog build:` reads the identity row back as well --
+`name='math3' unit=''` -- which is the pair the `.ui` holds the widgets for and the code fills from the
+channel being edited, so a row that stopped being the one the code writes shows there. Each line carries
+`(want ...)` and the reading
   taken from a real run is the reference:
   * `[math-probe] hit: inside node=0 which=-1, past-edge node=0 which=-1 (want 0 -1 0 -1)` — the
     output square straddles the box edge, so a hit test that recovers the node by a box test misses