## Why
A fork's inherited history can become unreadable when the preferred ancestor rollout is truncated or has an incomplete compressed frame, even when a usable archived copy exists.
## What changed
- Retry failed ancestor reads using another copy of the same rollout, validating its owner and location before rebuilding the lineage.
- Apply fallback to model-context loading, fork and revert preparation, and paginated history reads. Preserve the original error when no usable replacement exists.
- Validate inherited byte offsets when opening ancestors for consumption, allowing a newer model-context checkpoint to avoid reading an unusable ancestor.
- Limit ancestor materialization to the inherited history boundary while preserving any existing progress beyond it.
## Testing
Add regression coverage for incomplete compressed ancestors, replacement ownership checks, archived-copy recovery during resume and fork, and materialization boundaries. Add an app-server test that cold-resumes a fork and pages inherited turns and items from an archived copy when the active ancestor is too short.
GitOrigin-RevId: 43730c17329ac759f7292b16ddf0d9eb463e3310
Charlie Marsh
b7ca29d25b