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_report.py

@@ -17,14 +17,15 @@ DB = ComponentDatabase.load()
 def _feasible_config() -> CoilgunConfig:
     wire = next(w for w in DB.wires if w.part_id == "cu-petv2-1.0mm")
     capacitor = next(c for c in DB.capacitors if c.part_number == "cap-470uf-400v")
-    switch = next(s for s in DB.switches if s.part_number == "IRFP250PBF")
+    # мощный ключ (180А, surge ~720А) — конфигурация с реальным пиковым током
+    switch = next(s for s in DB.switches if s.part_number == "IRFP4468PBF")
     sensor = next(s for s in DB.sensors if s.part_number == "TCST2103")
     steel = next(m for m in DB.projectile_materials if m.name == "Ст3 (конструкционная сталь)")
 
     projectile = ProjectileConfig(material=steel, diameter_m=0.008, length_m=0.02)
     stage = StageConfig(
         wire=wire, capacitor=capacitor, switch=switch, sensor=sensor, tube_od_m=0.01,
-        turns_per_layer=20, layers=4, sensor_to_coil_distance_m=0.02, charge_voltage_v=350.0,
+        turns_per_layer=20, layers=4, sensor_to_coil_distance_m=0.02, charge_voltage_v=200.0,
     )
     return CoilgunConfig(
         stages=[stage, stage], inter_stage_gaps_m=[0.05], projectile=projectile,