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.

docs/claude/invariants-readouts-and-cursors.md

@@ -130,3 +130,14 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   stages the badge texts, which leave with the rest of the over batch over everything. An
   assertion on badge pixels inside a label's rows is the regression test: with the fill under
   the text, the label's glyph pixels are still lit inside the badge's rect.
+
+- **The instruments have one owner: the main window's row set.** The two cursors, the gate band,
+  the trigger marker and the measurement marks are *shared*, not copied per window. The main tick's
+  `sync_scope_instruments()` writes the owner's state into every scope window each frame (the
+  setters early-out, so a steady frame is a handful of comparisons), and a gesture inside a scope
+  window is read back from **that** window's controllers first -- `sync_cursors()`/`sync_gate()`
+  only adopt while their own plot holds the grab, which is exactly "the drag happened here" -- and
+  adopted by the owner before anything is written out. The measurement marks travel by **signal
+  name** (`RowSet::MeasureMark::signal`), so a window finds the row that plots it; a window that
+  does not plot the signal draws none. The search marks are times on the window and are placed on
+  every window by the same per-frame path.