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.

src/ui_app.hpp

@@ -2108,6 +2108,13 @@ void remember_closed_window(App& app, const App::ScopeWindow& scope);
 /// Opens a window from a remembered state: an empty scope window, then its rows, weights and
 /// signals. Returns the opened window (nullptr when it could not be made).
 App::ScopeWindow* open_scope_window_from_state(App& app, const LayoutState::Window& state);
+/// Writes the main window's instruments -- the two cursors, the gate band, the trigger marker --
+/// into every scope window, and adopts one dragged *there* back into the main window first. Called
+/// once per frame from the main tick, after the main's own `sync_cursors()`/`sync_gate()`.
+void sync_scope_instruments(App& app);
+/// The main window's measurement marks, translated to each scope window's own rows (by signal
+/// name) and placed there: a window that plots a measured signal draws its mark.
+void mirror_measure_marks(App& app);
 /// Refreshes the *Reopen closed windows* entry: its count and whether it is sensitive.
 void sync_reopen_scopes_item(App& app);
 /// Turns a scope window's own layout editor on or off, building or taking down its buttons.