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/skin.py

@@ -0,0 +1,78 @@
+"""Skin-effect corrections: frequency-dependent effective sheet
+resistance of a copper foil and via-barrel wall.
+
+1D diffusion through the foil thickness (exact): with tau = (1+j)/delta,
+the internal impedance per square of a foil of thickness t is
+
+    one-sided field (plane over a return plane):  Zs = tau*rho * coth(tau*t)
+    two-sided field (isolated foil):              Zs = tau*rho/2 * coth(tau*t/2)
+
+Both reduce to rho/t at DC and to rho/delta (resp. rho/(2*delta)) at
+high frequency. R_AC = Re(Zs) is used as the effective sheet resistance.
+
+HONESTY NOTE (also in the README): only the through-thickness current
+crowding is modeled. Lateral redistribution (proximity effect - AC
+current following the minimum-inductance path) needs a magneto-
+quasistatic solve and is NOT captured; since the resistance-driven
+distribution is the minimum-dissipation one, the reported AC resistance
+is a rigorous LOWER BOUND at the given frequency.
+"""
+from __future__ import annotations
+
+import cmath
+import math
+
+MU0 = 4e-7 * math.pi
+
+
+def skin_depth_m(freq_hz: float, rho_ohm_m: float) -> float:
+    return math.sqrt(2.0 * rho_ohm_m / (2.0 * math.pi * freq_hz * MU0))
+
+
+def _coth(x: complex) -> complex:
+    return 1.0 / cmath.tanh(x)
+
+
+def sheet_resistance_ac(thickness_m: float, freq_hz: float,
+                        rho_ohm_m: float, sides: int = 1) -> float:
+    """Effective sheet resistance [ohm/sq] of a foil at freq_hz.
+    sides=1: field on one side (plane facing a return plane, conservative);
+    sides=2: symmetric field on both sides (isolated foil)."""
+    if freq_hz <= 0.0:
+        return rho_ohm_m / thickness_m
+    delta = skin_depth_m(freq_hz, rho_ohm_m)
+    tau = (1.0 + 1.0j) / delta
+    if sides == 2:
+        zs = tau * rho_ohm_m / 2.0 * _coth(tau * thickness_m / 2.0)
+    else:
+        zs = tau * rho_ohm_m * _coth(tau * thickness_m)
+    return zs.real
+
+
+def resistance_factor(thickness_m: float, freq_hz: float,
+                      rho_ohm_m: float, sides: int = 1) -> float:
+    """R_AC / R_DC of a foil (or barrel wall) of the given thickness."""
+    if freq_hz <= 0.0:
+        return 1.0
+    return (sheet_resistance_ac(thickness_m, freq_hz, rho_ohm_m, sides)
+            / (rho_ohm_m / thickness_m))
+
+
+def parse_frequency(text: str) -> float:
+    """'0', '100k', '1.5M', '142500' -> Hz. Empty/invalid -> 0 (DC)."""
+    t = text.strip().lower().replace(",", ".").removesuffix("hz").strip()
+    if not t:
+        return 0.0
+    mult = 1.0
+    if t.endswith("meg"):
+        mult, t = 1e6, t[:-3]
+    elif t.endswith("m"):
+        mult, t = 1e6, t[:-1]
+    elif t.endswith("k"):
+        mult, t = 1e3, t[:-1]
+    elif t.endswith("g"):
+        mult, t = 1e9, t[:-1]
+    try:
+        return max(0.0, float(t) * mult)
+    except ValueError:
+        return 0.0