ui: the layout editor's buttons keep their look in one place, closing that plan row

The row asked for these buttons in a .ui file and that is not possible: there is one per
row and one more than that per boundary, and a GtkBuilder instantiates a file's objects
once, so the file can hold neither the count nor a prototype for the code to copy. The
plan's note ("only the buttons are template") is wrong for the same reason, and the plan
document now says so.

What was real in the row was the duplication: the look was written out three times -- the
icon, the two CSS classes that make a small round on-screen-display control, the size,
the overlay append and the bookkeeping -- and is now one function,
`layout_overlay_button()`. Each loop keeps only what makes it different: its alignment,
its sensitivity, the data it carries and, for the resize handle, its drag gesture.

The layout probe is the check and its readings are byte-identical before and after, all
seven lines: `buttons=5 done=1 done_box=[680,4,72x42] close=[4] add=[0,832] resize=[(1:-)]`
and the row geometry through add-above, remove, add-at-end and Editor off.

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

@@ -74,6 +74,12 @@ 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 |
 
+**The layout editor's buttons are not a `.ui` row, and the note above is wrong about them.** Their count *is*
+the row count (one per row, one per boundary), and `GtkBuilder` instantiates a file's objects once, so a file
+can hold neither the number nor a prototype for the code to copy. What was real in that row was the
+duplication: the shared look is now one function (`layout_overlay_button()`), and the layout probe reads
+byte-identical before and after. See `invariants-gtk-and-chrome.md`.
+
 **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