feat: portable graph export()/import() (BackupData v1) on brain.data()

brain.data() now serializes part or all of a brain to a versioned, portable
JSON document (BackupData) and restores it — the path for partial backups,
cross-environment moves, and 7.x→8.0 migration.

- export(selector?, options?): select by ids, collection (+transitive Contains),
  connected neighbourhood, vfsPath subtree, predicate, or whole brain; structural
  and predicate selectors compose. Options: includeVectors, includeContent (VFS
  blobs), includeSystem, edges ('induced'|'incident'|'none').
- import(backup, options?): onConflict 'merge' (dedup-by-id) | 'replace' | 'skip';
  reembed 'auto' (re-embed from data when no vector carried) | 'never'; remapIds
  to clone a subgraph under fresh ids.
- BackupData v1: format/formatVersion/brainyVersion/createdAt/embedding/entities/
  relations/blobs?/danglingIds?/stats. Reserved fields (subtype, data, confidence,
  weight, service) top-level; metadata custom-only. Current-state, no generations.
- Replaces the prior flat-entity export() (dropped relations) with a graph-complete
  document. Distinct from brain.import(file) ingestion, which is unchanged.
- Export BackupData/BackupEntity/BackupRelation/ExportSelector/ExportOptions/
  ImportOptions/ImportResult/DataAPI from the package root. CLI `brainy export`
  now writes a BackupData document.

Guide: docs/guides/backup-and-export.md. Tests: tests/unit/api/data-backup.test.ts.

docs/augmentations/EXAMPLES.md

@@ -546,11 +546,9 @@ const appComponent = await brain.findOne({ type: 'ReactComponent', name: 'App' }
 const imports = await brain.getRelated(appComponent.id, 'Imports')
 console.log(`App component imports:`, imports.map(c => c.name))
 
-// Export diagram
-const diagram = await brain.export({
-  format: 'react-diagram'
-})
-console.log(diagram.diagram) // Mermaid diagram
+// Export the analyzed component graph as a portable BackupData document
+const graph = await brain.data().then(d => d.export({ type: 'ReactComponent' }))
+console.log(`Exported ${graph.stats.entityCount} components, ${graph.stats.relationCount} edges`)
 ```
 
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