checkpoint should also be counted as an op for UpdateTransaction

[zhanglei1949]

Currently, the checkpoint mechanism in UpdateTransaction performs a simple dump-and-reopen, which works correctly for existing functionality. However, to support the upcoming Copy-on-Write (COW) feature (#370), the COW instance must be installed during the commit phase. Without this change, a failed commit will result in an unrecoverable error state when COW is enabled. This issue tracks the necessary refactoring to make the checkpoint logic COW-compatible.

[github-actions]

🤖 Auto-triage: linked as sub-issue of #86 — Refactor UpdateTransaction (confidence: high).

Reason: The issue focuses on refactoring the UpdateTransaction commit phase to support COW, which falls under the transaction layer and COW scope of #86.

Auto-applied: labels → transaction

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