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/SOURCE.md

@@ -0,0 +1,12 @@
+Vendored snapshot of https://github.com/manvalan/ImpedenceFinder
+
+Commit: a0c8d0ec37c9a1b099082926e50a245778ec8d6e
+
+Included: closed-form core (`zsolver`, `geometry`, `planes`, `model`,
+`net_walk`, `net_analysis`, `report`).
+
+Excluded on purpose:
+- `gerber2ems_export.py`, `prepare_simulation.py`, `crop_board.py`,
+  `simulate_net.sh` (OpenEMS / field-solver export)
+- `board_model.py` and `plugin/` (pcbnew). Pinscope does not load KiCad's
+  Python; PCB ingest stays in `pinscopex.parsers_kicad_pcb`.