Adds `Duty::Screen` and a coordinator relation that gives each barrage one screening force, chosen by name from forces that are not bombarding, withdrawing or breaking. The screening force picks a non-scout body and scores it with `Screen`, whose `interpose` term puts it between the barrage and the enemy. Co-Authored-By: Claude Code <noreply@anthropic.com> Change-Id: Ifdca1f503262f0f15a7fd4e822891a0302ec2db0