perf(rollout): reduce and trace writer flushes

[apanasenko-oai]

Summary

Reduce redundant rollout writer flushes and make persistence latency attributable.

• Skip clean-writer filesystem flushes.
• Remove the redundant batch-level flush from the thread-store append path.
• Add queue, writer, recovery, serialization, and filesystem timing spans.
• Preserve retry and flush-materialization behavior.

This is the rollout-writer slice split out of #30632.

Validation

• just test -p codex-rollout passed 80 tests.
• just test -p codex-thread-store passed 89 tests.
• just fmt passed, with an unrelated justfile rewrite discarded.