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/conftest.py

@@ -0,0 +1,53 @@
+"""Synthetic fixtures for the pure engine — no board_model, no pcbnew."""
+from __future__ import annotations
+
+import pytest
+
+from impedancefinder.model import DielectricLayer, Point2D, Stackup, TraceSegment, ZonePolygon
+
+
+def rect(x0: float, y0: float, x1: float, y1: float) -> tuple[Point2D, ...]:
+    return (Point2D(x0, y0), Point2D(x1, y0), Point2D(x1, y1), Point2D(x0, y1))
+
+
+@pytest.fixture
+def stackup_4layer() -> Stackup:
+    return Stackup(
+        copper_layer_names=("F.Cu", "In1.Cu", "In2.Cu", "B.Cu"),
+        dielectrics=(
+            DielectricLayer("prepreg_top", 4.3, 0.15),
+            DielectricLayer("core", 4.4, 0.7),
+            DielectricLayer("prepreg_bottom", 4.3, 0.15),
+        ),
+        copper_thickness_mm=0.035,
+    )
+
+
+@pytest.fixture
+def full_ground_plane() -> ZonePolygon:
+    """A ground pour on In1.Cu with no voids, spanning the whole test area."""
+    return ZonePolygon(net="GND", layer="In1.Cu", outlines_mm=(rect(-5, -5, 50, 5),))
+
+
+@pytest.fixture
+def split_ground_plane() -> ZonePolygon:
+    """A ground pour on In1.Cu with a gap between x=4mm and x=6mm."""
+    return ZonePolygon(
+        net="GND",
+        layer="In1.Cu",
+        outlines_mm=(rect(-5, -5, 4, 5), rect(6, -5, 50, 5)),
+    )
+
+
+@pytest.fixture
+def clean_run_segment() -> TraceSegment:
+    return TraceSegment(net="SIG", layer="F.Cu", start=Point2D(0, 0), end=Point2D(10, 0), width_mm=0.2)
+
+
+@pytest.fixture
+def neckdown_segments() -> tuple[TraceSegment, ...]:
+    """A trace that narrows partway along its run."""
+    return (
+        TraceSegment(net="SIG", layer="F.Cu", start=Point2D(0, 0), end=Point2D(5, 0), width_mm=0.3),
+        TraceSegment(net="SIG", layer="F.Cu", start=Point2D(5, 0), end=Point2D(10, 0), width_mm=0.12),
+    )