refactor: rename BackupData → PortableGraph (the type is interchange, not a backup)

Parity with the 8.0 rename, backported to the 7.x line. The brain.data()
export()/import() document type was BackupData, but it is the portable, versioned
interchange representation of a graph (entities + relations + optional vectors),
NOT a backup — exported for transport between instances, versions, and products.
The actual backup is the native snapshot, so "Backup*" mis-signalled.

Rename every developer-visible symbol, JSDoc, comment and doc:
- BackupData→PortableGraph, BackupEntity→PortableGraphEntity,
  BackupRelation→PortableGraphRelation, BACKUP_FORMAT[_VERSION]→
  PORTABLE_GRAPH_FORMAT[_VERSION], internal toBackup* helpers→toPortableGraph*.
- src/api/DataAPI.ts, src/index.ts, src/cli/commands/core.ts, docs, and the test
  (renamed data-backup.test.ts → data-portable-graph.test.ts).

The on-the-wire `format` tag is also renamed 'brainy-backup' → 'brainy-portable-graph':
the export/import format was introduced in 7.32.0 and has not been adopted by any
consumer, so there are no stored documents to stay compatible with — a clean rename
beats carrying a legacy tag forward. No deprecated aliases (nothing to alias).

1505 unit green; build clean; data-portable-graph.test.ts 20/20.

docs/guides/framework-integration.md

@@ -540,11 +540,11 @@ export async function generateStaticProps() {
   })
   await brain.init()
 
-  // Build a search index — export the whole brain as a portable BackupData document
-  const backup = await brain.data().then(d => d.export())
+  // Build a search index — export the whole brain as a portable PortableGraph document
+  const graph = await brain.data().then(d => d.export())
 
   return {
-    props: { searchIndex: backup.entities }
+    props: { searchIndex: graph.entities }
   }
 }
 ```