fix(db): clean up retired segment directories on Windows

[JalinWang]

Closes #644.

• Release zvec-owned segment file handles before removing retired directories.
• Treat physical cleanup as best-effort after the segment is logically retired.
• Add regression tests for old-directory removal and Windows handle-blocked cleanup.

External handle case

In the uncommon case where another process, such as a backup or inspection tool, opens a segment file on Windows without FILE_SHARE_DELETE, Optimize() cannot immediately remove the old directory.

The optimization remains successful because the replacement segment and manifest are already committed. The leftover directory will be removed on a later read-write open by the recovery cleanup introduced in #674, after the external handle is released.

Linux normally does not encounter this case because an open or memory-mapped file can still be unlinked; its storage is reclaimed after the last reference is closed.

Tests: targeted collection_test cases pass on Windows.