scope probe: read the windows' own GL account, and drop into the empty window

`[scope-probe]` grows the two readings a "blank scope window" report needs before it can be
blamed: `empty_drop=`/`empty_slot0=` (the same drop onto the empty window, the user's own
path) and `gl copy=/gl empty=/gl detached ok= frames=` -- each window's adapter context
status and staged-frame count. A scope whose context failed draws nothing at all: no axes,
no traces, exactly the "blank plot widget, no x axis" report -- and that is a driver/context
finding, not a layout one. The detached window is left open by the probe, so a run's
screenshots can answer "does the detached figure draw" with pixels.

docs/claude/probe-readings.md

@@ -1469,7 +1469,10 @@ 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
+[scope-probe] drop=1 slot1='square' page_groups=2 empty_drop=1 empty_slot0='square'
+[scope-probe] detach=1 scopes=3 frows=1 attach=1 scopes_back=2 frows_back=2 slot='ramp'
+[scope-probe] gl copy=ok(67f) empty=ok(39f) empty_bound=1
+[scope-probe] gl detached ok=1 frames=42 rows=1 slot0='sine'
 ```
 
 - `count=2` is both windows having been created and retained; `windows=1` each being a visible
@@ -1484,9 +1487,17 @@ a defect in the copy. Measured output:
   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.
+  into a scope window without dragging it there. `empty_drop=1 empty_slot0='square'` is the
+  same drop onto the **empty** window — the user's own path, "open a scope window and drop a
+  signal into it" — landing in its row set. `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.
+- The fourth line is the windows' **GL account** (`gl copy=`/`gl empty=`): `ok(Nf)` is a context
+  that staged N frames, `err:` carries the adapter's failure message. A scope whose context
+  failed draws *nothing* — no axes, no traces, exactly the "blank plot widget" report — so
+  `err:` there is a driver/context finding, not a layout one, and `empty_bound=1` (the dropped
+  signal's channel, read a frame after the drop) with `ok(...)` together mean the window is
+  drawing; a blank window under those two readings is a compositor problem, not the app's.
 
 ## `[search-probe]`