Chain stages into a multi-stage coilgun; document an over-pull failure mode

- sim/coilgun.py: chains StageConfig results through a global coordinate,
  stops honestly on the first infeasible stage (with which stage and why),
  and computes overall efficiency = exit KE / total capacitor energy in
- Discovered while wiring up the chain: a high-capacitance single-stage test
  configuration let current stay high past the coil center, decelerating and
  reversing the slug (exit velocity negative). This is a real coilgun tuning
  failure mode (not a bug) since KE ~ v^2 doesn't care about direction and
  the energy balance still holds to ~1%. Kept it as an explicit regression
  test (test_overpowered_discharge_can_fling_slug_backward) instead of
  quietly picking parameters that hide it, and tightened the "kinetic energy
  increases" test to also require forward (exit_v > 0) motion.

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

tests/test_coilgun_chain.py

@@ -0,0 +1,107 @@
+import pytest
+
+from gausse.components.schema import (
+    CapacitorSpec,
+    ProjectileMaterialSpec,
+    SensorSpec,
+    SwitchSpec,
+    WireSpec,
+)
+from gausse.sim.coilgun import CoilgunConfig, run_coilgun
+from gausse.sim.stage import ProjectileConfig, StageConfig
+
+STEEL = ProjectileMaterialSpec(
+    name="steel", density_kg_m3=7850.0, mu_r=200.0, b_sat_tesla=1.8, price_per_kg=100.0, source="test"
+)
+PROJECTILE = ProjectileConfig(material=STEEL, diameter_m=0.008, length_m=0.02)
+CAPACITOR = CapacitorSpec(
+    part_number="c1", capacitance_uf=100.0, voltage_v=400.0, esr_ohm=0.05, max_current_a=300.0,
+    price=300.0, source="test",
+)
+WIRE = WireSpec(
+    part_id="w1", material="copper", gauge_mm=0.8, insulation_od_mm=0.85,
+    resistivity_ohm_m=1.68e-8, max_current_a=10.0, price_per_m=3.0, source="test",
+)
+SWITCH = SwitchSpec(
+    part_number="sw1", kind="MOSFET", max_current_a=200.0, max_voltage_v=500.0,
+    on_resistance_ohm=0.02, on_voltage_drop_v=None, turn_on_time_ns=50.0, price=50.0, source="test",
+)
+OPTICAL_SENSOR = SensorSpec(
+    part_number="s1", kind="optical", propagation_delay_ns=500.0, price=20.0, source="test"
+)
+WEAK_INDUCTIVE_SENSOR = SensorSpec(
+    part_number="s2", kind="inductive", propagation_delay_ns=0.0, price=5.0, source="test",
+    sensitivity_v_per_mps=0.001, threshold_v=1.0,
+)
+
+
+def _stage(sensor=OPTICAL_SENSOR, sensor_to_coil_distance_m=0.02):
+    return 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=sensor_to_coil_distance_m,
+        charge_voltage_v=350.0,
+    )
+
+
+def test_three_stage_chain_is_feasible_and_accelerates():
+    config = CoilgunConfig(
+        stages=[_stage(), _stage(), _stage()],
+        inter_stage_gaps_m=[0.05, 0.05],
+        projectile=PROJECTILE,
+        initial_x_m=-0.05,
+        initial_v_mps=5.0,
+    )
+    result = run_coilgun(config)
+    assert result.feasible, result.reason
+    assert len(result.stage_outcomes) == 3
+    assert result.exit_v_mps > 5.0
+    assert 0.0 < result.efficiency < 1.0
+
+
+def test_global_coil_centers_accumulate_gaps():
+    config = CoilgunConfig(
+        stages=[_stage(), _stage(), _stage()],
+        inter_stage_gaps_m=[0.05, 0.07],
+        projectile=PROJECTILE,
+        initial_x_m=-0.05,
+        initial_v_mps=5.0,
+    )
+    result = run_coilgun(config)
+    assert result.feasible, result.reason
+    centers = [o.global_coil_center_m for o in result.stage_outcomes]
+    assert centers == pytest.approx([0.0, 0.05, 0.12])
+
+
+def test_infeasible_stage_stops_chain_with_honest_reason():
+    # у слабого индукционного датчика порог 1В при чувствительности 0.001В/(м/с)
+    # требует скорости 1000 м/с, которой у снаряда не будет -> ступень 2 обязана провалиться
+    config = CoilgunConfig(
+        stages=[_stage(), _stage(sensor=WEAK_INDUCTIVE_SENSOR), _stage()],
+        inter_stage_gaps_m=[0.05, 0.05],
+        projectile=PROJECTILE,
+        initial_x_m=-0.05,
+        initial_v_mps=5.0,
+    )
+    result = run_coilgun(config)
+    assert not result.feasible
+    assert result.failed_stage_index == 1
+    assert "индукционный датчик" in result.reason
+    # ступень 2 (индекс 2) не должна была даже запуститься
+    assert len(result.stage_outcomes) == 2
+
+
+def test_mismatched_gap_count_raises():
+    with pytest.raises(ValueError):
+        CoilgunConfig(
+            stages=[_stage(), _stage()],
+            inter_stage_gaps_m=[0.05, 0.05],
+            projectile=PROJECTILE,
+            initial_x_m=-0.05,
+            initial_v_mps=5.0,
+        )

tests/test_stage_energy_conservation.py

@@ -14,7 +14,7 @@ STEEL = ProjectileMaterialSpec(
 )
 PROJECTILE = ProjectileConfig(material=STEEL, diameter_m=0.008, length_m=0.02)
 CAPACITOR = CapacitorSpec(
-    part_number="c1", capacitance_uf=1000.0, voltage_v=400.0, esr_ohm=0.05, max_current_a=300.0,
+    part_number="c1", capacitance_uf=100.0, voltage_v=400.0, esr_ohm=0.05, max_current_a=300.0,
     price=300.0, source="test",
 )
 OPTICAL_SENSOR = SensorSpec(
@@ -62,7 +62,7 @@ def test_stage_is_feasible_with_realistic_components():
 
 def test_energy_conserved_exactly_when_lossless():
     lossless_capacitor = CapacitorSpec(
-        part_number="c-lossless", capacitance_uf=1000.0, voltage_v=400.0, esr_ohm=0.0,
+        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",
     )
     result = _run(
@@ -87,3 +87,29 @@ def test_kinetic_energy_increases_for_approaching_slug():
     result = _run(_wire(), _switch())
     assert result.feasible, result.reason
     assert result.kinetic_energy_delta_j > 0
+    # рост кинетической энергии тут должен означать разгон ВПЕРЁД, а не
+    # отброс назад (см. test_overpowered_discharge_can_fling_slug_backward)
+    assert result.exit_v_mps > 0
+
+
+def test_overpowered_discharge_can_fling_slug_backward():
+    """Честная фиксация реального режима отказа, а не только "успешных" случаев.
+
+    Слишком большая ёмкость (медленный разряд относительно скорости снаряда)
+    держит ток высоким уже после того, как снаряд проходит центр катушки —
+    зона x>0 тянет назад (dL/dx<0), и снаряд может выйти с отрицательной
+    скоростью. Энергобаланс при этом всё ещё точен (КЭ ~ v^2 не зависит от
+    знака) — это подтверждает, что находка отражает реальную физику плохо
+    настроенного расстояния/ёмкости, а не ошибку в уравнениях.
+    """
+    overpowered_capacitor = CapacitorSpec(
+        part_number="c-big", capacitance_uf=1000.0, voltage_v=400.0, esr_ohm=0.05,
+        max_current_a=300.0, price=300.0, source="test",
+    )
+    result = _run(_wire(), _switch(), capacitor=overpowered_capacitor)
+    assert result.feasible, result.reason
+    assert result.exit_v_mps < 0
+    balance = (
+        result.energy_remaining_cap_j + result.energy_dissipated_j + result.kinetic_energy_delta_j
+    )
+    assert balance == pytest.approx(result.energy_in_j, rel=1e-2)