Compute barrier_loss = 0.5 × max(0, 0.05×health − q_gap)² on the host from cached q_gap_ema. Written to last_barrier_loss (already declared by A4) and surfaced in HEALTH_DIAG `barrier=...`. Scalar-only / no gradient. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>