Slice: Save and export from current RopeHead through causal checkpoint evidence #226

Goal

Make save/export read from the current rope head and record durable evidence without mutating text authority.

Acceptance criteria

• Save exports a projection from head H.
• Save writes bytes to the target file path.
• Save admits a causal anchor and RopeCheckpointFact.
• Save does not create RopeRewrite, RopeDiff, or a new RopeHead.