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.

README.md

@@ -129,7 +129,11 @@ rect plus a `kMaximizeRevealPx` of slack, per row) and is invisible otherwise, s
 decorated by controls nobody is reaching for; one click makes that row fill
 the plot area and the same button restores the whole arrangement; the plot's right-click menu
 offers the same as **Full screen this figure** / **Restore the layout**, and a layout of a single
-figure carries no button (there is nothing to enlarge *to*). It is a *view* — the row set
+figure carries no button (there is nothing to enlarge *to*). Beside the toggle, and with the
+same show-on-approach, a **detach** button moves the figure into a window of its own (removed
+from the main window's rows, which save the change); the detached window carries an **attach**
+button in the same corner — its figure returns to the main window's rows and the window closes.
+It is a *view* — the row set
 saves every row's state on the way in and re-applies it on the way out (`RowState` in
 `plot_host.*`), so nothing plotted moves and nothing is saved. A plot's right-click menu has
 **Open this plot in a new window**, which copies that figure — its cells, their width shares and