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.

src/index.ts

@@ -116,12 +116,12 @@ export {
 // Export version utilities
 export { getBrainyVersion } from './utils/version.js'
 
-// Export portable graph backup/restore API (brain.data())
+// Export portable graph export/import API (brain.data())
 export { DataAPI } from './api/DataAPI.js'
 export type {
-  BackupData,
-  BackupEntity,
-  BackupRelation,
+  PortableGraph,
+  PortableGraphEntity,
+  PortableGraphRelation,
   ExportSelector,
   ExportOptions,
   ImportOptions,