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_resistance/errors.py

@@ -0,0 +1,34 @@
+"""User-facing error hierarchy.
+
+Every UserFacingError message is shown both on stdout (KiCad status bar)
+and in a matplotlib error figure, so keep messages self-contained and
+actionable.
+"""
+
+
+class UserFacingError(Exception):
+    pass
+
+
+class ApiVersionError(UserFacingError):
+    pass
+
+
+class SelectionError(UserFacingError):
+    pass
+
+
+class CandidateError(UserFacingError):
+    pass
+
+
+class ElectrodeError(UserFacingError):
+    pass
+
+
+class ConnectivityError(UserFacingError):
+    pass
+
+
+class GridSizeError(UserFacingError):
+    pass