fix: the math editor's body was a child of the window, not of the box above it

Moving the editor's frame into math-editor.ui left `math_body` and `math_actions` as
<child> elements of the *window* rather than of `math_outer`, because those edits were
anchored on the file's tail and closed the outer box two elements early. A GtkWindow
takes exactly one default <child>, so nothing failed loudly: the body -- with the canvas
and the parameter form inside it -- was left parentless, visible but never mapped, 0x0,
drawing nothing. Two symptoms, one cause: the canvas is blank, and the window is sized
around the head row and the buttons alone.

Measured against the pre-move revision in a worktree, same probe, same environment:
before this fix `canvas=0x0 mapped=0 window=756x940`; after it `canvas=420x836 mapped=1
window=938x940`, which is the old revision's own reading exactly.

It survived because that slice's probe read the editor's *values* (nodes, gain, output,
name, unit) and nothing about its structure. Every other panel moved that day has a
structural reading -- `controls_inside=13/13`, `nested=1`, `labels=2 list=1` -- and none
of them was affected. `[math-probe] editor frame:` is that reading now: the canvas's
allocation, its mapped state, and whether the canvas and the name entry are inside the
dialog at all, delayed 500 ms because an allocation only exists after a layout. In the
broken build it read `0x0 mapped=0 in_dialog=0`.

check_ui passes both files: the XML was valid and every property type-checked.

src/ui/math-editor.ui

@@ -46,8 +46,6 @@
             </child>
           </object>
         </child>
-      </object>
-    </child>
     <child>
       <object class="GtkBox" id="math_body">
         <property name="orientation">horizontal</property>
@@ -80,5 +78,7 @@
         </child>
       </object>
     </child>
+      </object>
+    </child>
   </object>
 </interface>