Barrel contacts for vias/THT pads; configurable cap-drill threshold

- Selected vias and through-hole pads inject at the drill-wall ring on
  every spanned layer (both contact models), not the whole pad face,
  so the pad/pour spreading resistance is part of the result. Vias are
  now selectable as contacts.
- Soldered THT joints: the hole is modeled solder-filled (core in
  parallel with the plating, also for stitching THT barrels) and the
  pad face carries an average-thickness solder coat over the modeled
  copper (SOLDER_THICKNESS_UM).
- Vias with drills above a configurable threshold (dialog field,
  default CAP_MAX_DRILL_MM = 0.5) keep open mouths even with capping
  selected - the fab caps only small vias.
- Geometry dump schema v6: electrode barrel fields, cap_max_drill_nm.
- Verified against R = rho/(pi t)*acosh(d/2a) for two circular contacts
  on a sheet (+2.6% at h = 0.15 mm, a = 1 mm; uniform model above the
  equipotential one as required by the contact bracket).

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

fill_resistance/solver.py

@@ -178,8 +178,12 @@ def _barrel_links(stack: RasterStack, problem: Problem
             length = problem.layers[lb].z_nm - problem.layers[la].z_nm
             if length <= 0:
                 continue
-            r_dc = via.barrel_resistance(length, problem.rho_ohm_m,
-                                         problem.plating_nm)
+            # THT pads carry a soldered component lead: the hole is
+            # solder-filled, the core conducts in parallel with the plating
+            r_dc = via.barrel_resistance(
+                length, problem.rho_ohm_m, problem.plating_nm,
+                solder_rho_ohm_m=(problem.solder_rho_ohm_m
+                                  if via.kind == "pad" else None))
             links.append((vi, la, ia, ja, lb, ib, jb, r_dc))
     return links, dead