feat: the main window's skeleton is a UI file

src/ui/main-window.ui holds the window, its two panes, the plot column with the status line under it,
and the sidebar's scroller. What stays in code is what depends on the stream: the events pane, the two
plot areas, the sidebar's panels and the status popover, each put into a slot by id.

The header bar is deliberately not in the file. It is the window's own titlebar, every button on it is
wired to an action, and an AdwApplicationWindow refuses a titlebar outright -- a fatal Adwaita-ERROR
rather than a warning that leaves the window merely ugly -- so the window is a GtkApplicationWindow and
the header bar stays where it was.

gtk_paned_set_position() is the other thing that had to stay in code, and the reason is order rather
than shape: it clamps against its children *now*, so stating it before the sidebar's panels exist moved
the plot column by ~180 px.

The layout, events and box-zoom probes read what they read before this change, and the header reads
window_button_w=130 bar_w=756 as it always has -- checked by building the previous commit and running
the same probe against it, because the layout numbers in the notes turned out to be stale from an
earlier session. Five UI files now.

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

@@ -43,6 +43,10 @@ needs a probe that proves it *loads* and that its contents match whatever drives
 
 ## Already done
 
+- `src/ui/main-window.ui` -- the main window's **skeleton**: the window, its two panes, the plot column
+  with the status line under it, and the sidebar's scroller. The four subtrees that depend on the stream
+  (the events pane, the two plot areas, the sidebar's panels, the status popover) are still built by
+  code and put into the slots by id.
 - `src/ui/source-form.ui` -- the form a source is added or edited with: an `AdwDialog` of
   `AdwPreferencesGroup` rows, with its action in the header bar where the label says which mode it is in.
 - `src/ui/help.ui` -- the help window's chrome: an `AdwWindow` with a header bar and an
@@ -62,6 +66,6 @@ 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` (5774 lines) | the main window's **skeleton** only -- the `AdwApplicationWindow`, the header bar, the plot area's placeholder, the panes' containers, the status bar -- with the custom widgets, popovers and dialogs left in code | one file, one window: converting the skeleton is reviewable, converting the whole file is not. Do it in slices (header bar, then the panes, then the trigger panel, then the left panes) with the probe suite after each |
+| `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 |
 
 Nothing in this list should be done in one pass. Each row is its own change, verified on its own.