GraphAdjacencyIndex.flush() was a no-op — LSM MemTables were never
written to SSTables for datasets under the 100K auto-flush threshold.
This caused readdir, getRelations, and getDescendants to return empty
results after close + reopen.
Three fixes:
- LSMTree.get(): merge MemTable + SSTable results (data spans both
after flush, old early-return missed SSTable data)
- GraphAdjacencyIndex.flush(): actually flush all 4 LSM-trees
- GraphAdjacencyIndex.close(): close all 4 trees (was only closing 2)
Also: brain.close() and shutdown hooks now call close() on graphIndex,
HNSW index, and metadataIndex to release timers and file handles.
