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/brainy.ts

@@ -117,13 +117,13 @@ import * as fs from 'node:fs'
 import { Db, type DbHost, type HistoricalQueryHandle } from './db/db.js'
 import {
   importGraph,
-  isBackupData,
-  type BackupData,
+  isPortableGraph,
+  type PortableGraph,
   type ExportSelector,
   type ExportOptions,
   type ImportOptions,
   type ImportResult
-} from './db/backup.js'
+} from './db/portableGraph.js'
 import { GenerationStore } from './db/generationStore.js'
 import { isDeterministicEmbedMode } from './embeddings/deterministicEmbedMode.js'
 import { GenerationConflictError } from './db/errors.js'
@@ -5074,22 +5074,22 @@ export class Brainy<T = any> implements BrainyInterface<T> {
   }
 
   /**
-   * @description Serialize part or all of the brain into a portable `BackupData`
+   * @description Serialize part or all of the brain into a portable `PortableGraph`
    * document — sugar for `brain.now().export(selector, options)` (the current
    * generation). For a past generation use `(await brain.asOf(g)).export(...)`;
    * for a speculative state use `brain.now().with(ops).export(...)`.
    *
-   * Restore a `BackupData` with {@link Brainy.import} (it dispatches a backup to the
+   * Restore a `PortableGraph` with {@link Brainy.import} (it dispatches a backup to the
    * graph round-trip; a file/buffer to ingestion). Distinct from `db.persist()`
    * (native whole-brain snapshot with generation history).
    *
    * @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.export({ ids }, { includeVectors: true })
    */
-  async export(selector: ExportSelector = {}, options: ExportOptions = {}): Promise<BackupData> {
+  async export(selector: ExportSelector = {}, options: ExportOptions = {}): Promise<PortableGraph> {
     return this.now().export(selector, options)
   }
 
@@ -6743,7 +6743,7 @@ export class Brainy<T = any> implements BrainyInterface<T> {
    * - Reduced confusion (removed redundant options)
    */
   async import(
-    source: Buffer | string | object | BackupData,
+    source: Buffer | string | object | PortableGraph,
     options?: {
       format?: 'excel' | 'pdf' | 'csv' | 'json' | 'markdown' | 'yaml' | 'docx' | 'image'
       vfsPath?: string
@@ -6770,9 +6770,9 @@ export class Brainy<T = any> implements BrainyInterface<T> {
       }) => void
     } & Partial<ImportOptions>
   ): Promise<ImportResult | any> {
-    // Portable graph round-trip: a BackupData document (produced by export()) is
+    // Portable graph round-trip: a PortableGraph document (produced by export()) is
     // restored via ONE atomic transaction — distinct from foreign-file ingestion below.
-    if (isBackupData(source)) {
+    if (isPortableGraph(source)) {
       return importGraph(this, this.storage, source, options as ImportOptions)
     }