Vector index upgrade and rollback

The vector entry identity fix prevents one node's chunk, named or property vector from overwriting another node's vectors. It changes derived search indexes; the authoritative graph storage format is unchanged.

These rollback steps apply to the release/v1.4.4 storage layout. Develop or canary builds can include independent storage migrations; this search fix does not establish downgrade safety for those builds.

Before upgrading, stop the database cleanly and take the normal backup of its data directory. Start the fixed version with the same data directory. Search indexes written by 1.4.3 or an earlier 1.4.4 build are incompatible and are rebuilt from the graph's stored embeddings. Allow time and disk space for this rebuild. A cache entry already overwritten by a collision is recovered from the graph; embeddings absent from authoritative graph storage cannot be recovered by rebuilding indexes.

Derived format changes are .vec/.meta version 2 to 3, HNSW 1.2.0 to 1.3.0, IVF-PQ bundle 2 to 3 and vector snapshot 1.0.0 to 1.1.0. Build fingerprints also include vkey2. No embedding storage key prefix or storage version is changed. This fix does not adopt the storage layout proposed in #1285.

To roll back to 1.4.3 or a build preceding this fix:

1. Stop the fixed database cleanly.
2. Move the derived <dataDir>/search directory outside the data directory, or remove it after taking a backup. For custom search persistence paths, move their derived caches too: BM25, vectors, HNSW, IVF/IVF-PQ and build settings. Keep the authoritative graph files and WAL untouched.
3. Start the older binary with the same data directory and let it rebuild its own search caches. Writes made after upgrading remain in graph storage.

Do not copy the new search cache files into an older binary's cache directory. The older binary retains its original collision bug; rollback restores its previous behavior. Re-upgrading rebuilds collision-free indexes from the same authoritative embeddings. A database with search persistence disabled simply rebuilds its in-memory indexes at startup.

The regression fixture under pkg/search/testdata/vector-v143 was written by 1.4.3 itself. Upgrade verification opens that graph and its caches, recovers the overwritten entry, writes another node and reopens. The corresponding rollback probe runs the 1.4.3 reader after removing only the derived caches.