fix(examples): keep the glfw demo's region off the pan path, and print the multiplot's region geometry

Adding a grabbable region across the middle of the glfw demo turned the pan
gesture PLOT_DEMO_FOLLOW makes -- and a user's own centre drag -- into a region
drag, which that mode's self-check reported as 'distance behind only by the pan:
NO' and exit 1. The region now rides the left sixth of the live window, so a
press in the middle pans exactly as it always did, and the reason is written
where the bounds are placed.

The multiplot demo also prints its region's last-frame fill rect and bound
columns, so the exit frame's region pixels can be read against the geometry
Plot::layout() measured.

Report: .superpowers/sdd/p2-task-7-report.md

examples/glfw_demo.cpp

@@ -580,7 +580,12 @@ int main() {
       } else {
         graph.set_cursor_x(cursor_a, xr.min + 0.35 * xr.span());
         graph.set_cursor_x(cursor_b, xr.min + 0.65 * xr.span());
-        graph.set_region(region, xr.min + 0.40 * xr.span(), xr.min + 0.60 * xr.span());
+        // The region sits on the left sixth of the window, not across its middle: it is a
+        // grabbable handle, so a region over the centre would turn the pan gesture the
+        // `PLOT_DEMO_FOLLOW` script makes (and the pan a user makes) into a region drag.
+        // The two cursors stay where they are -- neither is within a grab radius of the
+        // centre either, which is why the script's pan has always been a pan.
+        graph.set_region(region, xr.min + 0.06 * xr.span(), xr.min + 0.18 * xr.span());
       }
     }