layout: detach a figure to its own window, and attach it back

Beside each figure row's full-screen toggle -- same corner, same show-on-approach reveal, same
one-figure/never-maximized rules -- a **detach** button moves the figure into a window of its
own: copied into a new scope window (titled "detached") and removed from the main window's rows,
which then save the change. The detached window is the one scope window with an **attach**
control: an overlay wrapping its GL area, a button in the top-right corner revealed by the same
pointer-proximity rule from the overlay's own motion controller, and one click appends the
figure back to the main window's rows (cells, weights, every slot's signal in its colour) and
closes the window.

`detach_figure()` / `attach_scope_window()`; the reveal and placement share the maximize
buttons' geometry and pointer state, per row. A detached window's rows participate everywhere a
scope window's do (drops, the signal page's *Plot on* groups), which the previous commit gave
them.

Probe: `[maximize-probe]` now counts the detach buttons (`detach_single=0` for one figure,
`detach=2` for two, absent while maximized) and reads their corner reveal
(`detach_near=1 detach_other=0`, the pointer parked on the detach button's own centre -- the
strictest zone of the pair). Real-pointer smoke: hover reveals the pair, clicking detach opens
the window, clicking attach closes it with the figure back. Docs: README, the Mallard layout
page, the probe readings, the layout invariant, and the tour's plot stop.

help/C/org.udpscope.app/layout.page

@@ -77,7 +77,10 @@
       nothing is saved, nothing is dropped, and the rows' traces, colours and cursors come back
       where they were. The plot's right-click menu offers the same as <gui>Full screen this
       figure</gui> (and <gui>Restore the layout</gui> while one is enlarged). A layout of a single
-      figure has nothing to enlarge *to*, so it carries no button.</p></item>
+      figure has nothing to enlarge *to*, so it carries no button.</p></item> Beside the toggle, shown the same way, a <gui>detach</gui> button moves the figure into a
+      window of its own — it leaves the main window's rows; the detached window carries an
+      <gui>attach</gui> button in the same corner, which returns the figure to the main window and
+      closes the window.
       <item><p>A plot's right-click menu has <gui>Open this plot in a new window</gui>: a second
       window showing the same stream through a copy of that figure (its cells, their widths and
       every trace), with its own axes following the main window's time window. The overflow menu's