fix(snapshot): preserve nodes_written across resume

[jolestar]

Overview

Resume path loses previously written node count.

Scope

• Preserve nodes_written from resume progress.
• Ensure SnapshotMeta node_count includes prior writes.
• Guard worklist_position/logging against underflow.

Acceptance Criteria

• Resumed snapshot's node_count equals pre-resume + new writes (or derived from snapshot db).
• Progress logs do not underflow.
• Test covers resume continuation.

[jolestar]

@holonbot

[github-actions]

Holon completed successfully.

Run: https://github.com/rooch-network/rooch/actions/runs/20802886477