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>

tests/test_capping.py

@@ -50,11 +50,9 @@ def test_cap_at_foil_thickness_is_identity():
     so the result equals the feature-off reference (a 'pad'-kind barrel,
     which skips rings and mouths) with the mouth fully inside copper."""
     r_cap, _ = _solve(_two_layer(capped=True, cap_um=70.0), 0.1)
-    ref = _two_layer(kind="pad")
-    # 'pad' barrels are solder-filled; kill the core so the reference
-    # barrel matches the via's plating-only resistance exactly
-    ref.solder_rho_ohm_m = 1e30
-    r_ref, _ = _solve(ref, 0.1)
+    # a bare 'pad' barrel (solder_filled defaults False) is plating-only,
+    # exactly like the via's
+    r_ref, _ = _solve(_two_layer(kind="pad"), 0.1)
     assert r_cap.R_ohm == pytest.approx(r_ref.R_ohm, rel=1e-9)