Model every THT pad hole: exact pad copper, lead conductor, DNP holes

- THT pad copper is now part of the conductor: the exact pad outline
  (incl. oblong/custom shapes) is fetched from KiCad once per pad and
  stamped onto every included layer (the outer shape stands in for
  inner rings). Annular rings bridge antipads, and joints land on real
  copper instead of only pour coverage.
- The internal lead conductor is modeled in every solder-filled hole:
  a cylinder of drill - THT_LEAD_CLEARANCE_MM (0.25 fab rule) with
  THT_LEAD_RHO_OHM_M (copper default; config for brass/steel leads),
  in parallel with the solder annulus and the plating.
- Drill mouths of THT pads: populated pads keep conducting mouth
  copper (stands in for the solder plug - conservative, the plug is
  worth ~200 um of copper equivalent); DNP pad holes are cut open on
  every layer like uncapped via mouths.
- Oblong pads: the coat uses the exact pad shape; the lead cone tapers
  within the inscribed circle (new pad_min_nm on ViaLink/Electrode) so
  the long axis is not overstated sideways.

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

fill_resistance/config.py

@@ -39,7 +39,14 @@ THT_LEAD_PROTRUSION_MM = 1.5    # clipped THT lead protrusion on the side
                                 # opposite the component: a solder cone of
                                 # this height at the drill wall (tapering to
                                 # zero at the pad edge) wraps the lead of
-                                # every soldered THT CONTACT. 0 = no cones
+                                # every populated THT pad. 0 = no cones
+THT_LEAD_CLEARANCE_MM = 0.25    # hole diameter minus lead diameter (fab
+                                # rule): a lead cylinder of drill - this
+                                # conducts inside every solder-filled hole
+THT_LEAD_RHO_OHM_M = 1.68e-8    # lead material resistivity: copper leads/
+                                # wires; brass ~6.4e-8, phosphor bronze
+                                # ~1.1e-7, copper-clad steel higher - raise
+                                # this if your components use such leads
 SKIN_SIDES = 1                  # skin-effect field config: 1 = plane facing a
                                 # return plane (conservative), 2 = isolated foil