Begin each synthetic step before its user surface message so the structural V2-to-V3 migration can reserve its protected system head without reordering historical events. Keep the workload size, frame partition, original chunk provenance, and every timing/heap budget unchanged.
Add a small real persistence migration/publication/reopen prerequisite that preserves all user and assistant content and original V0 bytes. Both this regression and bounded stderr-headline regression fail before the fix and pass afterward. Worker errors retain a bounded head and tail instead of hiding migration refusal behind a generic stack tail.
Validation: full Session-opening benchmark16/16 passed, including all six128MB completion cases; focused full type-aware lint passed. First Agent resume median178.7ms and reopen27.4ms are below unchanged450/100ms budgets.
Tianyi Cui