Flashback Neoforge Fixed

Prevent startup crashes when Flashback's ReplayUI is absent, renamed, or unavailable during early Minecraft window initialization. Preserve every valid NeoForge numeric registry mapping when a mod leaves an unmapped (-1) entry, instead of dropping the entire registry snapshot. Compose the client block-entity snapshot guard with legacy Create: Flashback / Create: Aeronautics Flashback mixins instead of failing on an exclusive redirect conflict.