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/claude/invariants-gtk-and-chrome.md

@@ -248,6 +248,12 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   answer writes a row, and the row is gone once the dialog closes. `PrefsForm::browse_cancel` is
   cancelled by the `closed` handler and replaced by the next chooser: one open picker at a time, and none
   that can call back into a cleared form. It is also how a probe takes the cancel path without a mouse.
+- **An `AdwApplicationWindow` refuses a titlebar, and refuses it by *aborting*.** `gtk_window_set_titlebar()`
+  on one is a fatal `Adwaita-ERROR` (`is not supported for AdwApplicationWindow`), not a warning that
+  leaves the window merely ugly. The main window is therefore a `GtkApplicationWindow` -- an Adw window
+  would want its header as an `AdwToolbarView`'s top bar instead -- and `build_toolbar()`'s `GtkHeaderBar`
+  stays the window's own titlebar, which is also what gives the window its controls: a `GtkHeaderBar`
+  draws them only as a titlebar.
 - **libadwaita's rows do not share one property set, and a `.ui` is where that shows.** An
   `AdwEntryRow` is an `AdwPreferencesRow`: it has a `title` and **no `subtitle`**. `AdwSpinRow` and
   `AdwSwitchRow` are `AdwActionRow`s and have both. A hint about an entry -- "empty = unicast" --

docs/claude/invariants-plot-and-layout.md

@@ -4,6 +4,14 @@ How `plot::Figure`/`plot::Plot` may and may not be driven, what a row's state be
 
 Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to learn; read the ones touching the area you are editing before you edit it.
 
+- **A `GtkPaned`'s position is stated after both its sides are filled**, and that is a fact about the
+  *function's order* rather than about the window's shape, which is why it is the one part of the
+  window skeleton that cannot live in `main-window.ui`. `gtk_paned_set_position()` clamps against what
+  its children ask for *now*: called before the sidebar's panels have been appended it is clamped
+  against an empty pane, and the divider lands wherever that left it. Moving the call into the `.ui`
+  moved the plot column by about 180 px, which the layout probe read as a different `done_box` and a
+  taller figure.
+
 - **`plot::Plot` cannot remove or retarget a trace.** Hence fixed trace slots created
   up front and rebinding via `SlotSeries`; a *name* change (legend text) is the one
   thing that forces a rebuild. Cursors are parked outside the axis range instead of