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/vendor_path.py

@@ -0,0 +1,14 @@
+"""Put vendored ImpedenceFinder on sys.path (closed-form package only)."""
+
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+VENDOR_DIR = Path(__file__).resolve().parents[1] / "vendor"
+
+
+def ensure_impedancefinder() -> None:
+    root = str(VENDOR_DIR)
+    if root not in sys.path:
+        sys.path.insert(0, root)