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>

backend/routers/impedance.py

@@ -0,0 +1,75 @@
+"""Standalone impedance calculator (no PCB, no findings)."""
+
+from __future__ import annotations
+
+from dataclasses import asdict
+from typing import Literal
+
+from fastapi import APIRouter, HTTPException
+from pydantic import BaseModel
+
+from backend.pinscopex.impedance import (
+    GeometryError,
+    TraceGeometry,
+    coupled_diff_z,
+    cpw_z0,
+    export_kicad_dru,
+    microstrip_z0,
+    solve_width,
+    stackup_targets,
+    stripline_z0,
+)
+
+router = APIRouter(tags=["impedance"])
+
+
+class ImpedanceRequest(BaseModel):
+    mode: Literal["trace", "stackup"] = "trace"
+    kind: Literal["microstrip", "stripline", "cpw", "diff"] | None = None
+    h: float
+    er: float
+    t: float = 0.035
+    w: float | None = None
+    s: float | None = None
+    target_z: float | None = None
+
+
+def _z_for_kind(kind: str, geo: TraceGeometry) -> dict:
+    if kind == "microstrip":
+        return {"z0": microstrip_z0(geo), "w_mm": geo.w, "s_mm": geo.s, "kind": kind}
+    if kind == "stripline":
+        return {"z0": stripline_z0(geo), "w_mm": geo.w, "s_mm": geo.s, "kind": kind}
+    if kind == "cpw":
+        return {"z0": cpw_z0(geo), "w_mm": geo.w, "s_mm": geo.s, "kind": kind}
+    if kind == "diff":
+        zodd, zeven, zdiff = coupled_diff_z(geo)
+        return {
+            "z0": None,
+            "zodd": zodd,
+            "zeven": zeven,
+            "zdiff": zdiff,
+            "w_mm": geo.w,
+            "s_mm": geo.s,
+            "kind": kind,
+        }
+    raise GeometryError(f"unknown kind {kind}")
+
+
+@router.post("/impedance")
+def compute_impedance(body: ImpedanceRequest):
+    try:
+        if body.mode == "stackup":
+            s = body.s if body.s is not None else 0.2
+            targets = stackup_targets(h=body.h, er=body.er, t=body.t, s=s)
+            return {
+                "targets": {k: asdict(v) for k, v in targets.items()},
+                "kicad_dru": export_kicad_dru(targets),
+            }
+        kind = body.kind or "microstrip"
+        w = body.w
+        if body.target_z is not None:
+            w = solve_width(kind, body.target_z, body.h, body.er, body.t, s=body.s)
+        geo = TraceGeometry(h=body.h, er=body.er, t=body.t, w=w, s=body.s)
+        return _z_for_kind(kind, geo)
+    except GeometryError as exc:
+        raise HTTPException(400, str(exc)) from exc