Graviton and scalar blocks

This fixes the normalization from XI-A's Hessian. The same sign reproduces its Newtonian comparison: for $H=q-\eta$ and a static positive source,

A coefficient $-2$ would invert a different Hessian. The Newtonian check is not permission to change the quadratic normalization.

The scalar equation operator is $P_m$, with inverse ${{\mathsf{E}}_{\mathsf{m}}}^{\pm }$. Its action Hessian is $-P_m$, with inverse $-{{\mathsf{E}}_{\mathsf{m}}}^{\pm }$. This distinction determines the scalar causal bracket below.