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_zsolver.py

@@ -0,0 +1,80 @@
+"""zsolver validation: a published reference point plus monotonicity checks
+against the underlying physics, so the tests don't just re-derive whatever
+the implementation happens to compute.
+"""
+from __future__ import annotations
+
+import pytest
+
+from impedancefinder import zsolver
+
+
+def test_microstrip_matches_classic_50ohm_fr4_rule_of_thumb():
+    # ~3mm trace on 1.6mm FR4 (er~4.5) is the textbook "50 ohm microstrip"
+    # widely quoted in PCB fab application notes.
+    z0 = zsolver.microstrip_z0(width_mm=3.0, height_mm=1.6, er=4.5, t_mm=0.035)
+    assert z0 == pytest.approx(50.0, rel=0.05)
+
+
+def test_microstrip_z0_decreases_with_width():
+    narrow = zsolver.microstrip_z0(0.2, 0.15, 4.3, 0.035)
+    wide = zsolver.microstrip_z0(0.6, 0.15, 4.3, 0.035)
+    assert wide < narrow
+
+
+def test_microstrip_z0_increases_with_dielectric_height():
+    thin = zsolver.microstrip_z0(0.3, 0.1, 4.3, 0.035)
+    thick = zsolver.microstrip_z0(0.3, 0.3, 4.3, 0.035)
+    assert thick > thin
+
+
+def test_microstrip_z0_decreases_with_er():
+    low_er = zsolver.microstrip_z0(0.3, 0.15, 3.0, 0.035)
+    high_er = zsolver.microstrip_z0(0.3, 0.15, 5.0, 0.035)
+    assert high_er < low_er
+
+
+def test_microstrip_z0_finite_thickness_correction_is_a_small_effect():
+    with_thickness = zsolver.microstrip_z0(0.3, 0.15, 4.3, 0.035)
+    without_thickness = zsolver.microstrip_z0(0.3, 0.15, 4.3, 0.0)
+    assert without_thickness == pytest.approx(with_thickness, rel=0.15)
+
+
+def test_stripline_requires_positive_copper_thickness():
+    with pytest.raises(ValueError):
+        zsolver.stripline_z0(0.15, 0.3, 4.4, 0.0)
+
+
+def test_stripline_z0_decreases_with_width():
+    narrow = zsolver.stripline_z0(0.1, 0.5, 4.4, 0.035)
+    wide = zsolver.stripline_z0(0.3, 0.5, 4.4, 0.035)
+    assert wide < narrow
+
+
+def test_stripline_z0_increases_with_plane_spacing():
+    tight = zsolver.stripline_z0(0.15, 0.3, 4.4, 0.035)
+    loose = zsolver.stripline_z0(0.15, 0.6, 4.4, 0.035)
+    assert loose > tight
+
+
+def test_diff_microstrip_is_between_single_ended_and_twice_single_ended():
+    single = zsolver.microstrip_z0(0.2, 0.15, 4.3, 0.035)
+    diff = zsolver.diff_microstrip_z0(0.2, 0.15, 0.2, 4.3, 0.035)
+    assert single < diff < 2 * single
+
+
+def test_diff_microstrip_approaches_twice_single_ended_as_spacing_grows():
+    single = zsolver.microstrip_z0(0.2, 0.15, 4.3, 0.035)
+    wide_gap = zsolver.diff_microstrip_z0(0.2, 0.15, 5.0, 4.3, 0.035)
+    assert wide_gap == pytest.approx(2 * single, rel=0.02)
+
+
+def test_diff_stripline_approaches_twice_single_ended_as_spacing_grows():
+    single = zsolver.stripline_z0(0.15, 0.3, 4.4, 0.035)
+    wide_gap = zsolver.diff_stripline_z0(0.15, 0.3, 5.0, 4.4, 0.035)
+    assert wide_gap == pytest.approx(2 * single, rel=0.02)
+
+
+def test_cpwg_is_not_implemented():
+    with pytest.raises(NotImplementedError):
+        zsolver.cpwg_z0()