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).

docs/claude/probe-readings.md

@@ -1474,6 +1474,7 @@ a defect in the copy. Measured output:
 [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
+[scope-probe] pin_mirror=1 main=[-1.000..1.000] scope=[-1.000..1.000] x=[-1.000..1.000]
 ```
 
 - `count=2` is both windows having been created and retained; `windows=1` each being a visible
@@ -1493,6 +1494,12 @@ 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.
+- `pin_mirror=1` is a scope window **mirroring a pin as a range**: the probe pins the main window
+  and reads the scope's pin and its plot's x range a frame later. Both must agree and the x range
+  must be ordered (`x=[min..max]`, `min < max`). A `0` with an inverted scope pin (`[+pre, +post]`)
+  is the defect this exists for: `pin_event_window()` takes **spans**, and passing it a range's
+  signed `min` turned the window inside out, so a scope in Held or Paused mode drew no trace and no
+  x labels at all.
 - `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.