merge: 落库改 unnest,甩掉 PG bind 上限 → test

单批上限 500 → 20000。事务边界一字未动(仍是一个事务、三个并发守卫、RETURNING 照旧),
超时改成随条数放宽(30 秒打底 + 每条 3ms,上限 180 秒)。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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