Follow-up to #9530 and #9558.
The shard-level segment manifest was written inside the `segments/`
directory (`segments/manifest.json`). Older versions of Qdrant choke on
an unknown file inside `segments/`, so move it next to the directory as
`segments_manifest.json` instead.
- `SEGMENT_MANIFEST_FILE` is now `segments_manifest.json` and
`segment_manifest_path()` points at the shard root.
- The manifest is added to `ShardDataFiles` so clear/move handle it.
- Snapshots write the manifest to the snapshot root (next to
`segments/`), and restore/partial-snapshot loaders no longer need to
skip it inside `segments/`.
Co-authored-by: Cursor <cursoragent@cursor.com>
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