Splits fail with WAL

[udnay]

Note

Low Risk
Low risk: adds a new test case only, exercising shard cloning after close when WAL is configured in a separate directory.

Overview
Adds a new factory_tests case that verifies ShardFactory::clone_closed_shard succeeds after closing a parent shard that was opened with a separate WAL backend, and that both cloned children contain the parent’s data.

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[udnay]

This change is part of the following stack:

• Splits fail with WAL #218 ◀

_{Change managed by git-spice.}