Derivative support and surface non-cancellation

The convention for $\delta (t)$ here is the inherited surface distribution: it pairs with the trace on $\Sigma$. No additional factor or boundary normalization is introduced.

Lemma lem:no-cancel does not prove that $b{|}_{\Sigma }$ is nonzero. It proves that, once nonzero trace is established with the stated bulk regularity, the bulk term cannot remove its support. These are distinct obligations.