Smooth adaptive potential maps and draw the mesh on the raster figure

Two display fixes for the adaptive grid, from field feedback:

- Equipotential contours showed leaf-sized staircase corners on plane
  interiors: the potential is now expanded piecewise-LINEARLY from each
  leaf's reconstructed gradient instead of constant-per-leaf, and the
  default ADAPTIVE_MAX_CELL_UM drops 2 mm -> 1 mm (interior leaves
  beyond that buy almost nothing).
- The raster map now overlays the adaptive mesh: boundaries of coarse
  leaves draw in darker copper (fine regions stay plain = fully
  resolved), with a legend entry. Uniform-grid runs are unchanged.

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

fill_resistance/raster.py

@@ -46,6 +46,8 @@ class RasterStack:
                                             # copper-thickness factor (via
                                             # mouths: cap-thin or partially
                                             # drilled cells); None = all 1
+    mesh: np.ndarray | None = None      # bool (L, ny, nx): adaptive leaf
+                                        # boundaries (drawn on the raster map)
 
     @property
     def nlayers(self) -> int: