X72-51: Second-Order Invisible-Manifold Correction and the Viscous-Curvature Obstruction

Building on the two non-Beltrami source-hidden circles left by Round 50 (first-order state and source both hidden, yet the visibility carrier itself is nonzero), this round solves the corresponding second-order state-correction equation. Proves that although the correction term can be solved explicitly within the minimal two-sideband correction class, the central second-order source contains a viscous-curvature term independent of the correction's degrees of freedom, one that is nonvanishing on both source-hidden circles — so the minimal correction class cannot achieve second-order source cancellation. Route halts at STOP-C55 (viscous-curvature / coupled-Floquet-rescue gap): this round explicitly states that it has not ruled out a possible rescue from the homogeneous kernel of higher-sideband coupling, so this is only a negative result for the minimal correction class, not a complete second-order impossibility theorem — handed to the next round to test whether the hidden kernel can provide such a rescue; this round also includes a numerical-verification script.