scope windows take drops, and the signal page lists them

The second windows were read-only by accident: nothing accepted a drop on them, and the
signal page's *Plot on* buttons knew only the main window's rows.

- **Drop target per scope window**: the same string payload a badge drags (the signal index),
  handled by `on_scope_drop_channel()` -- the target carries its own window, the row is that
  window's cell under the pointer, and `assign_signal_to_scope()` lands the signal in the
  window's own row set (enabling an unacquired signal first, exactly as the main drop does).
  The rebuild and the area's re-point are the window's next frame in the tick: a drop arrives
  from inside the very widget it rearranges, and a synchronous rebuild would reset the drag
  controller under it.
- **Plot on, per window**: the signal page carries one named group per scope window ("In
  \"plot 2\""), one button per plot of that window, sensitive on that window's own rows. A
  window closing refreshes the pages in its destroy handler, before the deferred delete could
  dangle a button's scope pointer.
- Probe: `[scope-probe]` drops through the wired handler (`drop=1 slot1='square'`) and counts
  the page's groups (`page_groups=2`); the section documents that it must not run with
  `UDPSCOPE_ENABLE_ALL_MS` -- the enable-all probe's second leg clears the front plot, which
  is the slot this probe copies. README, help and the layout invariant follow.

docs/claude/probe-readings.md

@@ -1453,10 +1453,14 @@ places), arranged as two single-cell rows with one signal each. The two reads af
 
 `UDPSCOPE_PROBE_SCOPE_MS`. The **second window**: `open_scope_window()` twice — once copying
 the main window's figure 0, once empty — and read back a few frames later (the main tick is the
-scope's bind/acquire/repaint pump). Measured output:
+scope's bind/acquire/repaint pump). **Do not run it with `UDPSCOPE_ENABLE_ALL_MS`**: the
+enable-all probe's second leg, two seconds in, clears the front plot through the signal page --
+exactly the slot this probe copies -- so `copied=''` under that combination is that race and not
+a defect in the copy. Measured output:
 
 ```
 [scope-probe] count=2 copied='sine' bound=1 empty_ok=1 windows=1
+[scope-probe] drop=1 slot1='square' page_groups=2
 ```
 
 - `count=2` is both windows having been created and retained; `windows=1` each being a visible
@@ -1466,6 +1470,14 @@ scope's bind/acquire/repaint pump). Measured output:
   model, not a snapshot taken at creation.
 - `empty_ok=1` is the `-1` call producing an empty row, which is the other entry point the
   overflow menu uses.
+- The second line is the **window as a drop target and a *Plot on* group**: `drop=1` is
+  `on_scope_drop_channel()` — the handler the window's own target wires — putting `square` in
+  the copied window's second slot (`slot1=`), the rebuild and the area's re-point then left to
+  the window's next frame; `page_groups=2` is the signal page of an unplotted signal carrying
+  **one *Plot on* group per window** (`udpscope-scope-plot-on`), so a signal can be plotted
+  into a scope window without dragging it there. `drop=0` names a refused drop (a full plot or
+  an unacquirable signal); `page_groups` less than the window count is a page rebuilt before
+  the windows were read or a group that lost its window.
 
 ## `[search-probe]`