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/architecture/storage-architecture.md

@@ -457,23 +457,19 @@ await brain.storage.withLock('resource-id', async () => {
 
 ### Export Data
 ```typescript
-// Export entire database
-const backup = await brain.export({
- format: 'json',
- includeVectors: true,
- includeIndexes: false
-})
+// Export the whole brain to a portable BackupData document
+const backup = await brain.data().then(d => d.export(undefined, { includeVectors: true }))
 ```
 
 ### Import Data
 ```typescript
-// Import from backup
-await brain.import(backup, {
- mode: 'merge', // or 'replace'
- validateSchema: true
-})
+// Restore a BackupData document (dedup-by-id merge by default)
+await brain.data().then(d => d.import(backup, { onConflict: 'merge' }))
 ```
 
+See the [Export & Import guide](../guides/export-and-import.md) for partial exports
+(by id, collection, connected neighbourhood, VFS subtree, or predicate).
+
 ### Storage Migration
 ```typescript
 // Migrate between storage types
@@ -483,9 +479,9 @@ const newBrain = new Brainy({ storage: { type: 's3' } })
 await oldBrain.init()
 await newBrain.init()
 
-// Transfer all data
-const data = await oldBrain.export()
-await newBrain.import(data)
+// Transfer all data via a portable backup
+const data = await oldBrain.data().then(d => d.export(undefined, { includeVectors: true }))
+await newBrain.data().then(d => d.import(data))
 ```
 
 ## Performance Tuning