X72-19: Coupled Confluence of Middle-Strain and Quotient Amplitude

Continuing from Round 18's obstruction confluence loop, this round opens no new representation, instead directly coupling the critical quotient amplitude r=|v| with the middle-strain/determinant geometry λ₂⁺, (-det S)₊. It proves that the dangerous determinant-production rate is two-sided equivalent to λ₂⁺|S|², and that λ₂⁺≤|Sn| holds for all directions, so dangerous middle-strain cannot escape by choice of direction; it then defines the confluence ratio χ_C=λ₂⁺/|v| and proves the overlap–degeneracy inequality P₊²≲E_M·I₀, showing that dangerous activity has only two options: 'overlap with the quotient amplitude' or 'escape into low-amplitude degeneracy.' The route halts at STOP-C23 (confluence-ratio / low-amplitude-degeneracy gap), passing the baton to the next round to dissect directly the one remaining escape channel, |v|≈0.