Integrate ImpedenceFinder closed-form Z0 into the project Impedance tab.

Use the vendored Hammerstad-Jensen/Cohn engine for microstrip, stripline, and coupled-diff advice. Skip OpenEMS/pcbnew and refuse CPWG rather than inventing a number.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/impedancefinder/test_net_walk.py

@@ -0,0 +1,64 @@
+from __future__ import annotations
+
+import pytest
+
+from impedancefinder import net_walk
+from impedancefinder.model import Point2D, TraceSegment
+
+
+def test_pitch_must_be_positive():
+    with pytest.raises(ValueError):
+        net_walk.sample_net((), pitch_mm=0.0)
+
+
+def test_bend_chains_into_one_continuous_branch():
+    # Two segments sharing an exact endpoint at (5, 0) -- a bend, not a via.
+    first = TraceSegment(net="SIG", layer="F.Cu", start=Point2D(0, 0), end=Point2D(5, 0), width_mm=0.2)
+    second = TraceSegment(net="SIG", layer="F.Cu", start=Point2D(5, 0), end=Point2D(5, 5), width_mm=0.2)
+    branches = net_walk.sample_net((first, second), pitch_mm=1.0)
+    assert len(branches) == 1
+    distances = [s.distance_along_net_mm for s in branches[0].samples]
+    assert distances == sorted(distances)
+    assert distances[0] == 0.0
+    assert distances[-1] == pytest.approx(10.0)  # 5mm + 5mm, continuous
+
+
+def test_via_like_layer_change_stays_continuous():
+    # A segment on F.Cu ending exactly where a segment on In1.Cu begins --
+    # this is what a via looks like geometrically, with no ViaSpan needed
+    # for net_walk to treat it as one continuous run.
+    top = TraceSegment(net="SIG", layer="F.Cu", start=Point2D(0, 0), end=Point2D(3, 0), width_mm=0.2)
+    bottom = TraceSegment(net="SIG", layer="In1.Cu", start=Point2D(3, 0), end=Point2D(7, 0), width_mm=0.2)
+    branches = net_walk.sample_net((top, bottom), pitch_mm=1.0)
+    assert len(branches) == 1
+    assert branches[0].samples[-1].distance_along_net_mm == pytest.approx(7.0)
+
+
+def test_t_junction_splits_into_three_branches_zeroed_at_the_junction():
+    junction = Point2D(0, 0)
+    spoke_a = TraceSegment(net="SIG", layer="F.Cu", start=junction, end=Point2D(3, 0), width_mm=0.2)
+    spoke_b = TraceSegment(net="SIG", layer="F.Cu", start=junction, end=Point2D(0, 4), width_mm=0.2)
+    spoke_c = TraceSegment(net="SIG", layer="F.Cu", start=Point2D(-5, 0), end=junction, width_mm=0.2)
+    branches = net_walk.sample_net((spoke_a, spoke_b, spoke_c), pitch_mm=1.0)
+    assert len(branches) == 3
+    lengths = sorted(branch.samples[-1].distance_along_net_mm for branch in branches)
+    assert lengths == pytest.approx([3.0, 4.0, 5.0])
+    # every branch must start at the junction, not at its far leaf
+    assert all(branch.samples[0].distance_along_net_mm == 0.0 for branch in branches)
+
+
+def test_disconnected_segments_become_separate_branches():
+    isolated_a = TraceSegment(net="SIG", layer="F.Cu", start=Point2D(0, 0), end=Point2D(2, 0), width_mm=0.2)
+    isolated_b = TraceSegment(net="SIG", layer="F.Cu", start=Point2D(100, 0), end=Point2D(103, 0), width_mm=0.2)
+    branches = net_walk.sample_net((isolated_a, isolated_b), pitch_mm=1.0)
+    assert len(branches) == 2
+    lengths = sorted(branch.samples[-1].distance_along_net_mm for branch in branches)
+    assert lengths == pytest.approx([2.0, 3.0])
+
+
+def test_zero_length_segment_produces_a_single_sample():
+    point_segment = TraceSegment(net="SIG", layer="F.Cu", start=Point2D(1, 1), end=Point2D(1, 1), width_mm=0.2)
+    branches = net_walk.sample_net((point_segment,), pitch_mm=1.0)
+    assert len(branches) == 1
+    assert len(branches[0].samples) == 1
+    assert branches[0].samples[0].distance_along_net_mm == 0.0