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- 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>