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/invariants-plot-and-layout.md

@@ -357,7 +357,14 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   (same display mode, same epoch), and its span follows the main window's window or pin every frame. It is
   therefore a second *view* -- the settle rule is that a scope must never acquire, plan or epoch anything of
   its own. Closing the main window destroys its scopes first: a leftover toplevel keeps the GApplication's
-  loop alive around a window that is gone (the exit run's hang).
+  loop alive around a window that is gone (the exit run's hang). What a user *arranges* in a scope is the
+  scope's own arrangement, and it takes the same two doors the main window's rows take: a badge **dropped on
+  a scope window's plot** lands in that window's row set on the cell under the pointer (its own drop target,
+  its own `assign_signal_to_scope()`), and the signal page's *Plot on* section carries **one named group per
+  window** -- rebuilt out of `App::scopes` on every fill, and again when a window closes (the destroy handler
+  refreshes the pages before the deferred delete can dangle a button's scope pointer). The change itself is
+  never applied synchronously: a drop arrives from inside the very widget it rearranges, so the rebuild and
+  the area's re-point are the window's next frame in the tick.
 
 - **A figure row's full-screen is one click from two places, and never from one figure.** The
   toggle sits on the row's top-right corner, **out of hiding only while the pointer is near that