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/solver.py

@@ -178,13 +178,16 @@ def _barrel_links(stack: RasterStack, problem: Problem
             length = problem.layers[lb].z_nm - problem.layers[la].z_nm
             if length <= 0:
                 continue
-            # populated THT pads carry a soldered component lead: the hole
-            # is solder-filled, the core conducts in parallel with the
-            # plating (DNP pads and vias stay plating-only)
+            # populated THT pads carry a soldered component lead: lead
+            # cylinder (drill minus the fab clearance) + solder annulus
+            # in parallel with the plating (DNP pads and vias stay
+            # plating-only)
             r_dc = via.barrel_resistance(
                 length, problem.rho_ohm_m, problem.plating_nm,
                 solder_rho_ohm_m=(problem.solder_rho_ohm_m
-                                  if via.solder_filled else None))
+                                  if via.solder_filled else None),
+                lead_nm=max(via.drill_nm - problem.tht_lead_clearance_nm, 0),
+                lead_rho_ohm_m=problem.tht_lead_rho_ohm_m)
             links.append((vi, la, ia, ja, lb, ib, jb, r_dc))
     return links, dead