Follow-up hardening on the salvaged merge-into-trailing-turn fix:
- Merge only into REAL user tails (_is_real_user_message probe). Merging
into scaffolding tails (continuation marker, summary-as-user handoff)
would upgrade them to real-user evidence after SessionDB projection
strips the flags, breaking zero-user provenance (#69292 -
_is_synthetic_compression_user_turn keys on the TODO_INJECTION_HEADER
content marker, which merge-at-tail would bury mid-content).
- Strip a previously merged snapshot block before re-injection so
repeated boundaries refresh rather than accumulate todo state, and
refresh a bare stale snapshot row in place instead of stacking a
duplicate (empty/stale-skip semantics from #26981 by @YLChen-007).
- Scaffolding tails keep the flagged standalone append (pre-#53890
status quo; adjacent user rows are repaired downstream by
repair_message_sequence / _merge_consecutive_roles).
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