Initial import: KiCad zone resistance plugin

DC/AC resistance, power dissipation, and via/injection-area currents of
copper zone fills. KiCad 10 IPC-API plugin (kicad-python/kipy):
multi-layer via-coupled FDM solver, multi-part terminals via User.1/User.2
marker layers, pads as contacts, uniform-injection and equipotential
contact models, per-foil skin effect, optional solder/copper buildup on
mask openings. 54-case test suite incl. exact analytic references.

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

tests/test_skin.py

@@ -0,0 +1,103 @@
+"""Skin-effect model tests. The single-layer AC solve scales ALL in-plane
+conductances identically, so R_AC = R_DC * resistance_factor EXACTLY -
+which turns the analytic foil formula into an end-to-end exact test."""
+import math
+
+import numpy as np
+import pytest
+
+from fill_resistance import raster, skin, solver
+from tests.util import NM, make_multilayer, strip_problem
+
+RHO = 1.68e-8
+
+
+def _solve(problem, h_mm, i_test=1.0, freq=0.0):
+    stack = raster.rasterize_stack(problem, h_mm * NM)
+    e1, e2 = raster.electrode_masks(stack, problem)
+    return solver.run_solve(problem, stack, e1, e2, i_test, freq,
+                            contact_model="equipotential"), stack
+
+
+def test_skin_depth_value():
+    # copper @ 1 MHz: ~65-66 um
+    assert skin.skin_depth_m(1e6, RHO) * 1e6 == pytest.approx(65.2, rel=0.01)
+
+
+def test_sheet_resistance_dc_limit():
+    t = 70e-6
+    assert skin.sheet_resistance_ac(t, 0.0, RHO) == pytest.approx(RHO / t)
+    # low frequency: within 0.1% of DC
+    assert skin.sheet_resistance_ac(t, 100.0, RHO) == pytest.approx(
+        RHO / t, rel=1e-3)
+
+
+def test_sheet_resistance_high_f_limits():
+    t = 70e-6
+    f = 1e9                                  # delta << t
+    delta = skin.skin_depth_m(f, RHO)
+    assert skin.sheet_resistance_ac(t, f, RHO, sides=1) == pytest.approx(
+        RHO / delta, rel=0.01)
+    assert skin.sheet_resistance_ac(t, f, RHO, sides=2) == pytest.approx(
+        RHO / (2 * delta), rel=0.01)
+
+
+def test_resistance_factor_monotonic():
+    t = 70e-6
+    factors = [skin.resistance_factor(t, f, RHO)
+               for f in (0, 1e4, 1e5, 1e6, 1e7, 1e8)]
+    assert all(b >= a - 1e-12 for a, b in zip(factors, factors[1:]))
+    assert factors[0] == 1.0
+
+
+def test_parse_frequency():
+    assert skin.parse_frequency("") == 0.0
+    assert skin.parse_frequency("0") == 0.0
+    assert skin.parse_frequency("142k") == 142_000.0
+    assert skin.parse_frequency("1.5M") == 1_500_000.0
+    assert skin.parse_frequency("2meg") == 2_000_000.0
+    assert skin.parse_frequency("100000") == 100_000.0
+    assert skin.parse_frequency("100 kHz") == 100_000.0
+    assert skin.parse_frequency("junk") == 0.0
+
+
+def test_single_layer_ac_scales_exactly():
+    """Uniform conductance scaling leaves the field shape unchanged:
+    R_AC = R_DC * factor to solver precision."""
+    p = strip_problem(length=50, width=10, e_len=5)
+    f = 2e6                                    # delta=46um < t=70um
+    r_dc, _ = _solve(p, 0.5)
+    r_ac, _ = _solve(p, 0.5, freq=f)
+    factor = skin.resistance_factor(70e-6, f, RHO, sides=1)
+    assert factor > 1.2                        # real crowding at 2 MHz
+    assert r_ac.R_ohm == pytest.approx(r_dc.R_ohm * factor, rel=1e-9)
+    assert r_ac.rs_ratios[0] == pytest.approx(factor, rel=1e-12)
+    assert r_ac.skin_depth_um == pytest.approx(
+        skin.skin_depth_m(f, RHO) * 1e6, rel=1e-12)
+
+
+def test_via_chain_ac_exact():
+    """1D chain: layers scale by the foil factor, the barrel by the
+    plating-wall factor - exact composition."""
+    STRIP = [(0, 0), (10, 0), (10, 1), (0, 1)]
+    p = make_multilayer(
+        [[(STRIP, [])], [(STRIP, [])]],
+        rect1_mm=(0, 0, 1, 1), rect2_mm=(9, 0, 10, 1),
+        contact1="L0", contact2="L1",
+        vias_mm=[(5.5, 0.5)], gap_mm=1.0)
+    f = 2e6
+    sig_ac = 1.0 / skin.sheet_resistance_ac(70e-6, f, RHO, sides=1)
+    via_factor = skin.resistance_factor(18e-6, f, RHO, sides=2)
+    r_via_dc = RHO * 1e-3 / (math.pi * 0.3e-3 * 18e-6)
+    r_exact = (5 + 4) / sig_ac + r_via_dc * via_factor
+    res, _ = _solve(p, 1.0, freq=f)
+    assert res.R_ohm == pytest.approx(r_exact, rel=1e-9)
+
+
+def test_dc_default_unchanged():
+    """freq omitted -> identical to the pre-skin behavior."""
+    p = strip_problem(length=50, width=10, e_len=5)
+    res, _ = _solve(p, 0.5)
+    assert res.freq_hz == 0.0
+    assert res.skin_depth_um is None
+    assert all(r == 1.0 for r in res.rs_ratios)