Add eddy-current losses in the slug -- the missing loss channel

The user caught the evolution reporting 83.9% efficiency, which is
unphysical (real coilguns are single-digit %). Root cause: the model's
only loss channel was copper resistance; iron had no losses at all, so
the reluctance force came "for free" and the optimizer climbed into that
corner.

Adds eddy-current loss: the solid steel slug acts as a shorted secondary
(1-turn transformer), and its reflected resistance R_eddy = (wM)^2/R_e
is added to the circuit while the slug is inside the coil (x overlap(x)).
Energy now honestly goes to slug heating instead of kinetic. The formula
matches the classical solid-cylinder eddy loss (P ~ sigma*w^2*B^2*a^4),
so it's physically grounded, not tuned to a target. Wired through
schema/JSON (slug resistivity), losses.py, circuit.py, stage.py, and the
GPU batch integrator; energy conservation still holds (0.025%).

Effect: best efficiency 83.9% -> ~47%, and the distribution is now
realistic (median ~0%, most configs single-digit). 47% is still an
optimistic ceiling -- it's the optimizer's single best exploit, and the
model still omits tube friction, air drag, skin-effect field penetration
(skin depth ~0.44mm < slug radius), and timing imperfection. Hysteresis
computed but not in the dynamics (negligible, ~1e-4 J vs eddy). 89 tests.

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

tests/test_stage_energy_conservation.py

@@ -38,7 +38,7 @@ def _switch(on_resistance_ohm: float = 0.02) -> SwitchSpec:
     )
 
 
-def _run(wire, switch, capacitor=CAPACITOR):
+def _run(wire, switch, capacitor=CAPACITOR, projectile=PROJECTILE):
     # turns_per_layer выбран так, чтобы длина катушки (~1.7см) была сравнима
     # со снарядом (2см) — иначе снаряд, войдя глубоко внутрь длинной катушки,
     # оказывается в плоской зоне перекрытия (dL/dx=0) и сила не действует.
@@ -53,7 +53,7 @@ def _run(wire, switch, capacitor=CAPACITOR):
         sensor_to_coil_distance_m=0.02,
         charge_voltage_v=350.0,
     )
-    return run_stage(entry_x_m=-0.05, entry_v_mps=5.0, stage=stage, projectile=PROJECTILE)
+    return run_stage(entry_x_m=-0.05, entry_v_mps=5.0, stage=stage, projectile=projectile)
 
 
 def test_stage_is_feasible_with_realistic_components():
@@ -66,8 +66,16 @@ def test_energy_conserved_exactly_when_lossless():
         part_number="c-lossless", capacitance_uf=100.0, voltage_v=400.0, esr_ohm=0.0,
         max_current_a=300.0, price=300.0, source="test",
     )
+    # снаряд без вихревых потерь: огромное удельное сопротивление -> R_eddy≈0,
+    # чтобы «без потерь» действительно означало отсутствие всех каналов диссипации
+    no_eddy_steel = ProjectileMaterialSpec(
+        name="steel-no-eddy", density_kg_m3=7850.0, mu_r=200.0, b_sat_tesla=1.8,
+        price_per_kg=100.0, source="test", resistivity_ohm_m=1e12,
+    )
+    no_eddy_projectile = ProjectileConfig(material=no_eddy_steel, diameter_m=0.008, length_m=0.02)
     result = _run(
-        _wire(resistivity_ohm_m=0.0), _switch(on_resistance_ohm=0.0), capacitor=lossless_capacitor
+        _wire(resistivity_ohm_m=0.0), _switch(on_resistance_ohm=0.0),
+        capacitor=lossless_capacitor, projectile=no_eddy_projectile,
     )
     assert result.feasible, result.reason
     assert result.energy_dissipated_j == pytest.approx(0.0, abs=1e-9)