scope windows: a right-click menu of their own

A scope window is a view with its own arrangement, but its plot had no menu: every right-click
went nowhere, because the main window's row menu acts on the main window's rows.

`on_scope_right_click()` wires each scope window's GL area (beside its drop target) to a menu
that acts on **that window's** row set: y axis auto scale, y axis fixed range (the main menu's
dialog, aimed at the window's rows through `FixedRangeContext::rows`), fit, and one
*Remove '<signal>'* per trace on the plot. A detached window's menu also carries **Attach back
to the main window**. What it deliberately does not carry is an x fit: the window follows the
main window's time span, so an x fit here would be undone by the next frame's mirror.

`remove_trace()` and `FixedRangeContext` are generalized to take the row set they act on (the
main window passes its own). Probe: `[scope-probe] rclick=1` asserts the click gesture is wired
per window; the menu build is exercised by the real right-clicks the smoke makes.

docs/claude/probe-readings.md

@@ -1473,6 +1473,7 @@ a defect in the copy. Measured output:
 [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'
+[scope-probe] rclick=1
 ```
 
 - `count=2` is both windows having been created and retained; `windows=1` each being a visible
@@ -1492,6 +1493,9 @@ a defect in the copy. Measured output:
   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.
+- `rclick=1` is the scope window's **own right-click menu** being wired (one click gesture with
+  button 3 on its GL area, beside the drop target): the menu's actions act on that window's row
+  set.
 - 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