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_geometry.py

@@ -0,0 +1,88 @@
+import json
+import math
+
+import numpy as np
+
+from fill_resistance.geometry import (arc_points, linearize_ring,
+                                      load_problem, problem_from_json,
+                                      save_problem)
+from tests.util import make_multilayer, strip_problem
+
+
+def test_arc_points_quarter_circle():
+    r = 10_000_000  # 10 mm in nm
+    start = (r, 0)
+    mid = (int(r / math.sqrt(2)), int(r / math.sqrt(2)))
+    end = (0, r)
+    tol = 50_000  # 50 um
+    pts = arc_points(start, mid, end, tol)
+    assert len(pts) >= 3
+    radii = np.hypot(pts[:, 0].astype(float), pts[:, 1].astype(float))
+    assert np.allclose(radii, r, rtol=1e-6)
+    allpts = np.vstack([pts, [end]]).astype(float)
+    for a, b in zip(allpts[:-1], allpts[1:]):
+        half_chord = np.hypot(*(b - a)) / 2
+        sagitta = r - math.sqrt(max(r**2 - half_chord**2, 0.0))
+        assert sagitta <= tol * 1.01
+
+
+def test_arc_points_collinear_degrades_to_segment():
+    pts = arc_points((0, 0), (5_000_000, 0), (10_000_000, 0), 1000)
+    assert len(pts) == 1
+    assert tuple(pts[0]) == (0, 0)
+
+
+def test_linearize_ring_mixed_nodes_and_closure():
+    nodes = [
+        ("pt", (0, 0)),
+        ("pt", (10, 0)),
+        ("arc", ((10, 0), (17, 7), (10, 14))),
+        ("pt", (0, 14)),
+        ("pt", (0, 0)),
+    ]
+    ring = linearize_ring(nodes, tol_nm=1)
+    assert (ring[0] != ring[-1]).any()
+    assert len(ring) > 4
+
+
+def test_problem_json_roundtrip_v2(tmp_path):
+    p = make_multilayer(
+        [[([(0, 0), (10, 0), (10, 1), (0, 1)], [])],
+         [([(0, 0), (10, 0), (10, 1), (0, 1)], [])]],
+        rect1_mm=(0, 0, 1, 1), rect2_mm=(9, 0, 10, 1),
+        contact1="L0", contact2="L1", vias_mm=[(5.5, 0.5)])
+    f = tmp_path / "dump.json"
+    save_problem(p, f)
+    q = load_problem(f)
+    assert q.layer_names == ["L0", "L1"]
+    assert q.electrodes1[0].contact == "L0"
+    assert q.electrodes2[0].contact == "L1"
+    assert len(q.vias) == 1 and q.vias[0].drill_nm == p.vias[0].drill_nm
+    assert q.plating_nm == p.plating_nm
+    assert np.array_equal(q.layers[0].polygons[0].outline,
+                          p.layers[0].polygons[0].outline)
+    assert abs(q.sigma_s(0) - p.sigma_s(0)) < 1e-12 * p.sigma_s(0)
+
+
+def test_v1_schema_still_loads():
+    p = strip_problem()
+    v1 = {
+        "schema_version": 1,
+        "board_path": "old",
+        "layer_name": "F.Cu",
+        "net_name": "GND",
+        "thickness_nm": 70000,
+        "thickness_source": "stackup",
+        "rho_ohm_m": 1.68e-8,
+        "rect1": {"x0": 0, "y0": 0, "x1": 1000, "y1": 1000,
+                  "layer_name": "User.1"},
+        "rect2": {"x0": 5000, "y0": 0, "x1": 6000, "y1": 1000,
+                  "layer_name": "User.1"},
+        "polygons": [{"outline": p.layers[0].polygons[0].outline.tolist(),
+                      "holes": []}],
+    }
+    q = problem_from_json(json.loads(json.dumps(v1)))
+    assert q.layer_names == ["F.Cu"]
+    assert q.vias == []
+    assert q.electrodes1[0].contact == "all"
+    assert q.layers[0].thickness_nm == 70000