## Why
Guardian reviews must not lose user authorization constraints or reuse an
unreadable or incompatible parent checkpoint after compaction.
## What changed
- Retain bounded excerpts of oversized root user messages across compaction,
resume, and rollback, and recover excerpts from legacy Guardian history when
available.
- Require thread-owned Guardian review sessions and asynchronous scoring to
reuse only non-empty checkpoints produced by a compatible review model.
- Fail closed instead of reviewing without the required checkpoint when reuse
is disabled, compatibility is unknown, or checkpoint content is unusable.
## Testing
Expanded retained-context, review-session, asynchronous-scoring, and app-server
coverage for oversized instructions, legacy recovery, model incompatibility,
missing content, resume, rollback, and disabled checkpoint reuse.
GitOrigin-RevId: f30d837197e0d838e656a632109f490c38208f5e
4636819a35