Initial import: KiCad zone resistance plugin

DC/AC resistance, power dissipation, and via/injection-area currents of
copper zone fills. KiCad 10 IPC-API plugin (kicad-python/kipy):
multi-layer via-coupled FDM solver, multi-part terminals via User.1/User.2
marker layers, pads as contacts, uniform-injection and equipotential
contact models, per-foil skin effect, optional solder/copper buildup on
mask openings. 54-case test suite incl. exact analytic references.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

fill_res_action.py

@@ -0,0 +1,14 @@
+"""Entrypoint launched by KiCad when the toolbar button is pressed.
+
+Lives at the plugin root (next to plugin.json) and only bootstraps the
+package import path, so fill_resistance/ can use normal absolute imports.
+"""
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+
+from fill_resistance.main import main
+
+if __name__ == "__main__":
+    main()