Stage 2B: source switching is not density switching

This is Obstacle B, independent of (eq:B) XG,XFST. The difference is classical, supported in the onset layer for the commutator in (eq:symmetric), and can propagate retardedly. It is not a loop counterterm.

Off-shell failure of symmetry alone would not exclude a symmetric operator agreeing on the free shell. The next section supplies precisely that stronger test.