README: potential map as its own figure, |J| map back to plain

The overlaid contour lines belonged on the potential plot the plugin
already produces; demo-potential.png is that figure for the same demo
solve.

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

README.md

@@ -16,10 +16,12 @@ current**. PNGs + a text summary are saved per run.
 THT-pad contact (V+, injected at the drill-wall ring) squeezes past a
 notch in the F.Cu pour, transfers through the stitching-via field into
 the B.Cu pour and leaves at the V− lug — per-via currents and the
-hottest via are reported. The white equipotential lines share their
-levels across the layers, so their density reads as field strength:
-nearly the whole drop happens around the notch on F.Cu, only two lines
-fall across the B.Cu pour.*
+hottest via are reported.*
+
+![Potential on the two-layer demo net](docs/img/demo-potential.png)
+*The matching potential map with equipotential contour lines: they
+bunch where the field is strongest — nearly the whole 8.7 mV drop
+happens around the notch on F.Cu.*
 
 Uses the KiCad **IPC API** (`kicad-python` / `kipy`), not the deprecated
 SWIG API. Requires KiCad **10.0.1+**.