Clarify raw exchange metadata and streamline host tests

docs/development/raw_exchange.md

@@ -20,6 +20,14 @@ their geometric classification when their prescribed velocity is zero.
 All results are base-lattice units per unit out-of-plane depth. For local
 spacing and timestep h=dt, raw `(jx,jy,tz,dm)` converts to impulse
 `(h*dt*jx,h*dt*jy,h*h*dt*tz)` and mass `h*h*dm`.
+The accumulator intentionally accepts h and dt separately to make the spatial
+and temporal conversion explicit; the current acoustic hierarchy supplies dt=h.
+The stored rx/ry arms run from geometry.center to the actual wall hit, not to
+the fluid node. For this raw exchange j=c_i(a+b), moving the arm from the fluid
+node to the hit adds q*c_i, whose cross product with j is zero. Thus either arm
+gives the same raw torque (up to floating-point rounding); this equivalence
+must not be assumed for a future nonparallel stress/advective correction.
+
 Force/torque are impulse/angular impulse divided by the reported window length.
 `dm=b-a` is positive into the exterior fluid, while j is the opposite of the
 fluid's momentum change across that cut link. Interpolated BFL can introduce