fix: the sidebar keeps its own width, so a small window can no longer hide it

The sidebar's width was a GtkPaned divider position: `window - 940` in pixels. On any window
narrower than 940 px the divider landed past the right edge and the sidebar came up 0 px wide,
reachable only by dragging the divider back.

Every attempt that kept the paned failed for that reason -- deriving the position from the paned's
own width, re-parenting the column, property-form children, code-set size requests. The mechanism was
the fault, so it is gone: the row is an AdwOverlaySplitView, which is what a utility sidebar is in
GNOME, with min-sidebar-width = max-sidebar-width = 320 in pixels. Its width is therefore its own at
any window size, and when there is no room beside the plot it *overlays* the plot (collapsed, set from
the window's width in tick()) instead of being squeezed to nothing.

Two things found on the way, both recorded in the invariants:

- sidebar-width-fraction is a 0..1 fraction of the window whatever sidebar-width-unit says. Setting a
  pixel count there is a GObject critical and it falls back to 0.25, which is why the sidebar first
  read 560. The width is pinned with min/max instead.
- the window opened at its content minimum (756 px, the plot column plus the sidebar) because the
  explicit gtk_window_set_default_size() was dropped when the window moved into the .ui. Restored.

[sidebar-probe] reads both states and is the guard: window=756 sidebar=320 under a 2x scale, which is
the case that used to read 0.

docs/claude/probe-readings.md

@@ -1043,6 +1043,23 @@ library measures and draws the rect in tests of its own.
   defect this reading exists for, and it was invisible from the frame alone: the file view pinning its
   axis from the ring each frame is what swallowed the gesture.
 
+## `[sidebar-probe]`
+
+```
+[sidebar-probe] beside     window=756 sidebar=320 collapsed=1
+[sidebar-probe] overlaid   window=756 sidebar=320 collapsed=1
+```
+
+`UDPSCOPE_PROBE_SIDEBAR_MS` reads the sidebar's width in both of the split view's states: beside the plot,
+and **overlaid** on it (`collapsed`, which is the state a window with no room beside the plot produces).
+
+- **`sidebar` must never be 0.** It was, for every window narrower than the 940 px the divider used to sit
+  at, and the pane could only be found by dragging the divider back — the report this reading exists for.
+  It must read its own width (320) in *both* states: `collapsed` changes where the sidebar is, never how
+  wide it is.
+- `collapsed=1` on the `beside` line means the window is narrower than `kSidebarRoom` (900), which is the
+  expected reading on a scaled display, where the logical width is half what the screen looks like.
+
 ## `[prefs-probe]`
 
 ```