perf: coalesce Array Visualizer rebuilds and halve scatter restyle

In scatter mode a two-array (X/Y) plot rebuilt the table and chart once
per property setter (data, offset, stride) and restyled every marker four
times, so a ~4000-point phase portrait froze for ~0.8s per refresh.

- SeerArrayWidget: add a BulkUpdate RAII guard that defers create() until
  a burst of setters completes, so a refresh does one rebuild, not several.
- redecorateScatterSeries: drop the two redundant transparent passes; after
  addSeries() the series holds the theme pen/brush, so setPen(NoPen)/
  setBrush() are already real transitions and either re-pushes both to the
  markers.

~800ms -> ~200ms per refresh at 4000 points; line/spline and marker
appearance unchanged.

Add tests/helloscatter, a 4000-point two-array phase portrait with a
breakpoint loop, to reproduce the slow refresh.

tests/helloscatter/helloscatter.cpp

@@ -0,0 +1,49 @@
+#include <cstdio>
+
+#define NPOINTS 4000
+
+double pos[NPOINTS];
+double vel[NPOINTS];
+
+//
+// Fill pos/vel with the trajectory of a damped harmonic oscillator
+// (semi-implicit Euler integration).
+//
+void integrate (double damping) {
+
+    double x  = 1.0;
+    double v  = 0.0;
+    double w  = 1.0;
+    double dt = 0.005;
+
+    for (int i=0; i<NPOINTS; i++) {
+        v += (-w*w*x - damping*v) * dt;
+        x += v * dt;
+
+        pos[i] = x;
+        vel[i] = v;
+    }
+}
+
+//
+// Exercises the Array Visualizer with a large two-array scatter plot.
+//
+// Open the Array Visualizer, enter 'pos' as array A and 'vel' as array B,
+// length 4000, select B's axis as 'Y', scatter mode, and check 'Auto'.
+// Then set a breakpoint on the printf below and 'Continue' a few times:
+// each stop recomputes the trajectory with a different damping and
+// refreshes the 4000-point phase portrait.
+//
+int main (void) {
+
+    for (int run=0; run<10; run++) {
+
+        double damping = 0.05 + 0.05*run;
+
+        integrate(damping);
+
+        printf("run %d: damping=%.2f  pos[end]=%f\n", run, damping, pos[NPOINTS-1]);
+    }
+
+    return 0;
+}