Full solder joint at every populated THT pad, read from KiCad

No size-threshold guessing: whether a hole is a via or a THT pad
already comes from KiCad (board.get_vias vs drilled board.get_pads).
Every populated THT pad of the net now carries the complete joint the
contacts got: solder-filled barrel, average-thickness coat over a
pad-diameter disc on the outer layers, and the protruding-lead cone on
the side opposite its owning footprint. The footprint side and the
Do-not-populate flag are read from KiCad (footprint pads store absolute
positions, so owner lookup is an exact (x, y, number) map); DNP pads
stay plating-only with no joint. Contact pads are deduplicated by
barrel center so their cone/coat is never applied twice.

Barrels are now gathered in single-layer runs too: via rings and drill
mouths perforate a lone plane, THT joints stiffen it locally.

ViaLink gains solder_filled + protrusion_side (legacy dumps load with
the old every-THT-pad-filled semantics).

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

fill_resistance/solver.py

@@ -178,12 +178,13 @@ def _barrel_links(stack: RasterStack, problem: Problem
             length = problem.layers[lb].z_nm - problem.layers[la].z_nm
             if length <= 0:
                 continue
-            # THT pads carry a soldered component lead: the hole is
-            # solder-filled, the core conducts in parallel with the plating
+            # 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)
             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))
+                                  if via.solder_filled else None))
             links.append((vi, la, ia, ja, lb, ib, jb, r_dc))
     return links, dead