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>

vendor/impedancefinder/net_analysis.py

@@ -0,0 +1,62 @@
+"""Shared net-analysis orchestration used by both cli.py (one net, full
+per-sample detail) and board_report.py (many nets, summarized). Pure: works
+on an already-loaded BoardData/Stackup, no pcbnew import needed here, so
+it's testable with no KiCad installed.
+"""
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Optional
+
+from impedancefinder import geometry, net_walk
+from impedancefinder.model import BoardData, ImpedanceSample, NetBranch, Stackup, TraceSegment
+
+
+@dataclass(frozen=True)
+class NetAnalysisResult:
+    samples: tuple[ImpedanceSample, ...]
+    branch_count: int
+    partner_net_name: Optional[str]  # None means single-ended
+
+    @property
+    def is_differential(self) -> bool:
+        return self.partner_net_name is not None
+
+
+def analyze_net(
+    board_data: BoardData, stackup: Stackup, net_name: str, pitch_mm: float, single_ended: bool = False
+) -> NetAnalysisResult:
+    """Raises ValueError if the net has no routed segments on this board."""
+    segments = segments_for(board_data, net_name)
+    if not segments:
+        raise ValueError(f"no segments found on net {net_name!r}")
+    branches = net_walk.sample_net(segments, pitch_mm)
+    partner_net = None if single_ended else geometry.find_pair_net_name(net_name)
+    partner_segments = segments_for(board_data, partner_net) if partner_net else ()
+    if not partner_segments:
+        partner_net = None
+    results: list[ImpedanceSample] = []
+    for branch in branches:
+        results.extend(_analyze_branch(branch, partner_segments, stackup, board_data))
+    return NetAnalysisResult(samples=tuple(results), branch_count=len(branches), partner_net_name=partner_net)
+
+
+def _analyze_branch(
+    branch: NetBranch, partner_segments: tuple[TraceSegment, ...], stackup: Stackup, board_data: BoardData
+) -> tuple[ImpedanceSample, ...]:
+    if not partner_segments:
+        return tuple(
+            geometry.analyze_sample(sample, stackup, board_data.zone_polygons) for sample in branch.samples
+        )
+    return tuple(
+        geometry.analyze_differential_sample(sample, partner_segments, stackup, board_data.zone_polygons)
+        for sample in branch.samples
+    )
+
+
+def segments_for(board_data: BoardData, net_name: str) -> tuple[TraceSegment, ...]:
+    return tuple(segment for segment in board_data.segments if segment.net == net_name)
+
+
+def net_length_mm(board_data: BoardData, net_name: str) -> float:
+    return sum(segment.length_mm for segment in segments_for(board_data, net_name))