Overlay equipotential lines on the README current-density figure

Shared contour levels across both layers, so line density reads as
field strength (the caption points at the notch carrying nearly the
whole drop).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/gen_readme_figs.py

@@ -104,12 +104,32 @@ def demo_problem() -> Problem:
     return p
 
 
+def _overlay_potential(fig, res, stack):
+    """Equipotential contours on the per-layer current-density axes,
+    on shared levels so line density reads as field strength."""
+    vmax = float(np.nanmax(res.V))
+    vmin = min(0.0, float(np.nanmin(res.V)))
+    levels = np.linspace(vmin, vmax, 17)[1:-1]
+    ext = stack.extent_mm()
+    ny, nx = stack.shape2d
+    xs = np.linspace(ext[0], ext[1], nx, endpoint=False)
+    xs += (xs[1] - xs[0]) / 2
+    ys = np.linspace(ext[3], ext[2], ny, endpoint=False)
+    ys += (ys[1] - ys[0]) / 2
+    for li, ax in enumerate(a for a in fig.axes if a.images):
+        with np.errstate(invalid="ignore"):
+            ax.contour(xs, ys, res.V[li], levels=levels, colors="white",
+                       linewidths=0.5, alpha=0.65)
+
+
 def gen_demo_maps():
     p = demo_problem()
     res, stack, e1, e2 = _solve(p, 0.05)
+    fig_j = plots.fig_current(res, stack, e1, e2, p)
+    _overlay_potential(fig_j, res, stack)
     figs = [
         (plots.fig_raster(stack, e1, e2, p, res), "demo-raster"),
-        (plots.fig_current(res, stack, e1, e2, p), "demo-current"),
+        (fig_j, "demo-current"),
     ]
     plots.save_and_show(figs, OUT, show=False)