[TASK] Batched fsync implementation (P3)

[kirich1409]

Context

Buffer entries in memory; flush + fsync every 10-50 items or every 100 ms. Crash recovery: incomplete trailing line is treated as corrupt and skipped.

Acceptance criteria

• AC-31 passes: fsync count ≤ ceil(n/50) + 2 for n items.

Files to touch (estimate)

best-effort estimate; agent refines during implementation

Implementation notes

n/a

Dependencies

• Blocked by [TASK] RemovalLogger actor + JSONL append path #51

Safety checklist

See .github/ISSUE_TEMPLATE/task.yml — tick applicable invariants in the issue if any destructive path is touched.

Definition of Done

• Acceptance criteria satisfied and verifiable
• Tests added or updated
• No build / lint / test regressions
• Self-review against research-report invariants
• Code review approved