feat: the cursors panel is a UI file

src/ui/cursors-panel.ui holds the panel's shape -- the header with the mode dropdown and the reset
button, and the grid the two cursor times are read in. What is not in it is the grid's contents,
which follow the cursor mode and the rows and are rebuilt whenever either changes.

Panels need a builder that outlives them, unlike the dialogs, which are rebuilt on every opening: a
widget belongs to the builder that built it, so App::ui_builders holds each panel's for the life of
the window. ui_load() is that, in ui_widgets.cpp.

make test-ui also checks that udpscope-ui.gresource.xml lists every .ui file now. That was the third
trap of this refactor -- a file on disk but not in the resource is a Gtk-ERROR at load, which aborts
-- and it is a compile-time mistake that nothing else in the build could see. Verified by unlisting
the new file and watching the check fail, then restoring it.

[cursors-probe] reads as it did: all four modes, pane=1, the timings and the drag cases.

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

@@ -66,6 +66,15 @@ needs a probe that proves it *loads* and that its contents match whatever drives
 | where | what | why it is the right next step |
 |---|---|---|
 | `src/source_dialog.cpp` (the chooser's own window) | the chooser: `.ui` for the window, its two pages and its action row. Its **host form is done**; what is left is the list, the notebook and the buttons around it | the last hand-built UI outside `ui.cpp`, and the file is already open |
-| `src/ui.cpp` (the panels inside the skeleton) | the trigger panel, the signal panel, the cursors/measurements/math panels, the layout editor's overlay, the menus and the popovers: `.ui` for each, in that order, with the probe that reads it run after each | each is self-contained inside the skeleton that is now in place, and each has a probe of its own |
+| `build_measures_panel` (`src/ui.cpp`, ~82 lines) | the measurements panel: `.ui` + `tools/check_ui.py` after | small, self-contained, and its own probe |
+| `src/ui/ui_trigger.cpp` (~22 kB) | the trigger panel: the `.ui` holds the panel and its groups; the signal chips and the condition rows stay in code because they follow CONFIG | the largest user-visible panel |
+| `src/ui/ui_signals.cpp` (~20 kB) | the signals panel | mostly a list of data-driven rows, so the `.ui` holds the frame |
+| `src/ui/ui_math.cpp` (~48 kB) | the math panel and the node editor's chrome (the canvas itself is drawn, not declared) | the largest file, and the one where least of it is template |
+| `src/ui/ui_events.cpp` (~44 kB) | the events pane | the rows are events, the frame is not |
+| `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 |
+
+Each of these is one change on the pattern `src/ui/ui_cursors.cpp` now follows: `ui_load()` the file, resolve
+the ids, keep every value, signal and rebuild in code, and run the panel's own probe before committing.
 
 Nothing in this list should be done in one pass. Each row is its own change, verified on its own.