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.

README.md

@@ -138,6 +138,10 @@ and `PlotArea`; the overflow menu's **New scope window** opens one with an empty
 window is another *view* of the same acquisition: the main window's tick binds and acquires its
 rowset under the same model mutex, mirrors the main window's time window (or its pin) onto it
 every frame, and pumps its render — so the two windows never disagree about the data, the display
+mode or the span. **A signal badge dragged onto a scope window's plot lands there** — the same
+drop the main plot takes, into that window's own row set, on the cell the pointer is over — and
+the signal page's *Plot on* buttons list **every window**: one group per window (named, `In
+"plot 2"`), one button per plot of it.
 mode or the span.
 
 **The window.** The plot is the window. Under it sits a fixed-height **badge bar** (36 px). Its left