scope windows: mirror a pin as a range, and restore the tick rebuild

**The blank scope window in trigger mode.** A held event pins the main window to
`[-pre, +post]`, and the tick mirrors that pin onto every scope window. It passed
`pinned_range().min/max` to `pin_event_window()`, which takes **spans** (`pre, post`): `min` is
signed, so `pre` came out negative and the group range became `[+pre, +post]` — an inverted x
window. The plotter then had nothing to draw: no trace, no x-axis labels, an empty plot that
looked like a broken window. `RowSet::pin_display_range(x0, x1)` states the pin as the range a
mirroring view actually has, and the tick uses it; live mode was never affected, which is why the
defect only showed under the trigger.

Also restores the scope-window `rebuild_if_dirty()` in the tick, which a reproduction experiment
had left commented out (`REPRO-TEMP`) and a later commit carried: a window opened empty and filled
by a drop kept the figure it was built with.

Probe: `[scope-probe] pin_mirror=1 main=[..] scope=[..] x=[..]` pins the main window, reads the
scope's pin and plot x range a frame later, and reads `0` for an inverted or mismatched pin.
Docs: probe readings and the trigger/pinning invariant (range vs spans).

src/plot_host.hpp

@@ -570,6 +570,13 @@ class RowSet {
   /// 0, so nothing outside the pre+post window is visible.
   void pin_event_window(double pre_s, double post_s);
 
+  /// The same pin, stated as the **range** a second view mirrors: the caller has the display
+  /// range (`x0 < x1`) and not the spans -- a scope window takes `pinned_range()` from the main
+  /// window and hands it on. `pin_event_window()` takes *spans*, and passing it a range's signed
+  /// `min` turned the pin inside out (a scope in Held mode pinned to `[+pre, +post]`: no trace
+  /// and no x labels). A degenerate or inverted range pins nothing.
+  void pin_display_range(double x0, double x1);
+
   /// Fits every row's y axis to the data it holds, and puts x back on the whole time window --
   /// `fit_x()`'s x half exactly, so the two cannot drift apart. Like it, **this does not change
   /// the time window**: the y axes are fitted to their data, x is merely un-zoomed.