refactor(8.0): rename BackupData → PortableGraph (the type is interchange, not a backup)

The export()/import() document type was named BackupData, but it is not a backup:
it is the portable, versioned, partial-or-whole interchange representation of a
graph (entities + relations + optional vectors/blobs) — the unit Brainy exports
for transport between instances, versions, and products, and the payload other
artifacts embed. The actual backup is persist()/load() (the native whole-brain,
generation-preserving snapshot), so "Backup*" actively collided with that concept.

Rename every developer-visible symbol, file, doc, JSDoc and comment:
- BackupData→PortableGraph, BackupEntity→PortableGraphEntity,
  BackupRelation→PortableGraphRelation, BackupReader/Writer→PortableGraphReader/Writer,
  BackupValidation→PortableGraphValidation, isBackupData→isPortableGraph,
  validateBackup→validatePortableGraph, BACKUP_FORMAT[_VERSION]→PORTABLE_GRAPH_FORMAT[_VERSION].
- src/db/backup.ts → src/db/portableGraph.ts; test → db-portable-graph.test.ts.
- Guide, api/README, RELEASES updated.

The on-the-wire `format` tag is renamed 'brainy-backup' → 'brainy-portable-graph':
the 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. No deprecated aliases (nothing to alias). 7.x shipped the
same rename as 7.32.2.

1471 unit green; build clean; db-portable-graph.test.ts 17/17.

src/db/db.ts

@@ -57,8 +57,8 @@ import type {
   Result
 } from '../types/brainy.types.js'
 import type { StorageAdapter } from '../coreTypes.js'
-import { exportGraph } from './backup.js'
-import type { ExportSelector, ExportOptions, BackupData } from './backup.js'
+import { exportGraph } from './portableGraph.js'
+import type { ExportSelector, ExportOptions, PortableGraph } from './portableGraph.js'
 import {
   splitNounMetadataRecord,
   splitVerbMetadataRecord
@@ -416,7 +416,7 @@ export class Db<T = any> {
 
   /**
    * @description Serialize part or all of this database value into a portable
-   * `BackupData` document, read **as of this view's generation**. Because `export`
+   * `PortableGraph` document, read **as of this view's generation**. Because `export`
    * lives on the immutable `Db`, it composes with every way of obtaining one:
    * `brain.now().export(sel)` (current), `(await brain.asOf(g)).export(sel)`
    * (time-travel export), and `brain.now().with(ops).export(sel)` (what-if export).
@@ -427,11 +427,11 @@ export class Db<T = any> {
    *
    * @param selector - WHAT to export (omit for the whole brain). See {@link ExportSelector}.
    * @param options - HOW to export (vectors / VFS bytes / edge policy). See {@link ExportOptions}.
-   * @returns A versioned, portable `BackupData` document.
+   * @returns A versioned, portable `PortableGraph` document.
    * @example
    * const backup = await brain.now().export({ collection: id }, { includeVectors: true })
    */
-  async export(selector: ExportSelector = {}, options: ExportOptions = {}): Promise<BackupData> {
+  async export(selector: ExportSelector = {}, options: ExportOptions = {}): Promise<PortableGraph> {
     this.assertUsable('export')
     return exportGraph(this, this.host.storage, selector, options)
   }