Open tensor legs and source-contracted reduction

The derivatives ${{\partial }_y}^{\alpha }$ and ${{\partial }_y}^{\beta }$ arise from the differentiated source deltas in (eq:t-test). Their displayed sign is the distributional derivative-transfer sign.

In (eq:Vblock), derivatives act only on their designated edge. They do not act on $c$, ${\chi }_{\mathrm{I}}$, or $L$. This fixes the recipe before any extension or integration.