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/config.py

@@ -28,7 +28,13 @@ VIAS_CAPPED = True              # filled + capped vias (dialog checkbox):
                                 # Ring/pad copper of vias is modeled either
                                 # way; THT-pad copper/drills are not.
 CAP_PLATING_UM = 15.0           # cap plating thickness (fab spec)
-INCLUDE_TH_PADS = True          # plated through-hole pads stitch layers too
+CAP_MAX_DRILL_MM = 0.5          # fab caps only small vias: drills above this
+                                # stay open even with VIAS_CAPPED
+                                # (dialog-settable)
+INCLUDE_TH_PADS = True          # plated through-hole pads stitch layers too;
+                                # their holes are modeled solder-filled (a
+                                # soldered component lead), so the solder core
+                                # conducts in parallel with the plating
 SKIN_SIDES = 1                  # skin-effect field config: 1 = plane facing a
                                 # return plane (conservative), 2 = isolated foil