scope windows: the same instruments as the main window

A scope or detached window drew its traces and nothing else: no cursors, no gate band, no trigger
marker, no measurement marks. They are not per-window state to be recreated -- the cursors are one
pair of times, the gate one band, the trigger one instant -- so the main window's row set stays the
**owner** and every window is written from it.

`sync_scope_instruments()` runs in the main tick after the main's own `sync_cursors()`/
`sync_gate()`: a gesture inside a scope window is read back from *that* window's controllers first
(those two calls only adopt while their own plot holds the grab, which is exactly "the drag
happened here"), adopted by the owner, and then every window -- the dragged one included -- is
written from the owner with the same cursor pair, mode and visibility, gate bounds and visibility,
and trigger marker. `RowSet::set_cursors()` is the new pair setter; `trigger_marker_x()`/`_visible()`
are the getters the mirror reads. The measurement marks travel by signal name
(`RowSet::MeasureMark::signal`, filled by the measures pass), so each window finds the row that
plots the signal and draws its ruler or mark there -- or none, when it does not plot it. The search
marks are times on the window and are placed on every window by the same per-frame path, and the
crosshair-axis-labels preference is restated to open windows when it changes.

Probe: `[scope-probe] instruments windows=N cursors=N gate=N trigger=N marks=N (main trigger
on=..)`, after adding a real gated `Top` measurement on `sine` -- the gate is restated visible each
frame by `update_measures()` and marks come only from `show_on_plot` measurements, so the leg uses
the app's own path. Docs: README, the cursors invariant, and the probe readings.

README.md

@@ -151,6 +151,11 @@ you. The overflow menu's **Reopen closed windows (N)** brings them back, arrange
 window that was *attached* is not remembered (its figure is on the main window again), and a
 detached figure is preserved by its window's entry, so quitting with one open no longer loses it.
 
+A scope window draws the **same instruments as the main window** — the two cursors (draggable
+there, and shared: moving one in any window moves it everywhere), the gate band, the trigger
+marker and the measurements' marks on the traces each window actually plots. They have one owner
+(the main window's plot set) and are written into every window each frame, so no window can drift.
+
 A scope window also **carries its own layout editor**: `L` (or the grid button in its corner,
 revealed like the others) shows the same `+`/`×`/Done controls the main window's editor has, and
 they change *that* window's rows — so a second window can hold several figures as easily as the