Fix two physics bugs the user caught: unbounded field and ignored current limits

1. Iron saturation now in the dynamics (was the "13 Tesla" bug). Flux
   linkage lambda(x,I) = L_air*I + L_iron*overlap(x)*g(I) with
   g(I)=I_sat*tanh(I/I_sat) saturating at the current where iron reaches
   B_sat. Both the circuit back-EMF (dlambda/dx) and the force (coenergy,
   dW'/dx) derive from the SAME lambda, so energy stays conserved (0.025%
   error) AND the field caps at B_sat instead of running to 13 T. Reduces
   to the old 0.5*I^2*dL/dx in the low-current limit. On the config that
   reported 60% efficiency / 11.6 T, it now gives 40% / 1.90 T.

2. Switch surge-current limit is now a hard feasibility constraint: a
   config whose peak discharge current exceeds the switch's pulse rating
   (continuous * surge factor per device kind) is infeasible -- otherwise
   the optimizer "wins" with configs that vaporize their own thyristor
   (e.g. 119 A through a 12 A BT151). Capacitor current stays a warning
   (electrolytics tolerate pulses; DB has continuous not pulse ratings).

Regression tests added for both (field cap near saturation, energy
conservation under strong discharge). 87 tests pass.

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

tests/test_circuit_rlc_analytical.py

@@ -28,15 +28,12 @@ def _make_frozen_model() -> CoilInductanceModel:
 
 
 def _integrate(r_ohm: float, t_span: tuple[float, float], t_eval: np.ndarray):
+    # mu_eff=1.0 -> вклад железа нулевой, λ = L_air·I, чистый RLC-контур
     model = _make_frozen_model()
     params = StageCircuitParams(
         capacitance_f=C_F,
         r_total_ohm=r_ohm,
         mass_kg=1.0,
-        mu_eff=1.0,
-        total_turns=100,
-        coil_length_m=0.05,
-        b_sat_tesla=1e9,  # эффективно отключает клэмп насыщения для этого теста
     )
     state0 = [Q0, 0.0, 0.0, 0.0]
     return solve_ivp(