合法迁移
不是看两个端点各自合不合法，而是看它们之间有没有一条逐帧合法的路径。
每帧检查冲突，证书 ke-transition/1 记录帧哈希。
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English summary
A legal transition is a sequence of configurations where every frame passes hard checks. This demo shows 7 frames between two clean loop conformers (0 clashes in every frame). The certificate records each frame's SHA-256.