Completed transcript entries now write through to the OS page cache on
append instead of microtask-batching, supersede in-flight atomic rewrites
with a synchronous full-body publish, and land on the live moveTo path
(source pre-rename, destination post-rename) so a software crash no longer
drops finished user/assistant/tool events. Streaming text remains durable
only at message_end; no fsync/power-loss guarantee is claimed.