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.

RELEASES.md

@@ -80,7 +80,7 @@ historical records) are documented in:
 - [docs/ADR-001-generational-mvcc.md](docs/ADR-001-generational-mvcc.md) — the
   design record: persisted layout, commit protocol, crash recovery, proof table
 
-### Portable export & import (BackupData v1)
+### Portable export & import (PortableGraph v1)
 
 A portable, versioned graph format that serializes part or all of a brain to a
 single JSON document and restores it — the cross-environment, cross-version
@@ -88,29 +88,34 @@ single JSON document and restores it — the cross-environment, cross-version
 
 ```ts
 // Export is a method on the immutable Db, so it composes with now()/asOf()/with()
-const backup = await brain.export({ collection: id }, { includeVectors: true })
-await otherBrain.import(backup, { onConflict: 'merge' })  // dedup-by-id
+const graph = await brain.export({ collection: id }, { includeVectors: true })
+await otherBrain.import(graph, { onConflict: 'merge' })  // dedup-by-id
 
 ;(await brain.asOf(gen)).export(sel)        // time-travel export
 brain.now().with(ops).export(sel)           // what-if export
 ```
 
 - **`brain.export(selector?, options?)` / `db.export(...)`** → a versioned
-  `BackupData` document. Selectors reuse `find()`'s grammar: `{ ids }`,
+  `PortableGraph` document. Selectors reuse `find()`'s grammar: `{ ids }`,
   `{ collection }` (alias `memberOf`, transitive `Contains`),
   `{ connected: { from, depth } }`, `{ vfsPath }`, predicate
   (`{ type, subtype, where, service }`), or the whole brain (omit) — and they
   compose. Options: `includeVectors`, `includeContent` (VFS file bytes),
   `includeSystem`, `edges: 'induced' | 'incident' | 'none'`.
-- **`brain.import(backup, options?)`** is polymorphic: a `BackupData` document is
+- **`brain.import(graph, options?)`** is polymorphic: a `PortableGraph` document is
   restored as **one atomic transaction** (`onConflict: 'merge' | 'replace' | 'skip'`,
   `reembed: 'auto' | 'never'`, `remapIds` for cloning); a file/buffer routes to the
   existing CSV/PDF/Excel/JSON ingestion. No migration for ingestion callers.
-- **`validateBackup(data)`** — dry-run structural/version/endpoint check before import.
-- **Format** (`format:'brainy-backup'`, `formatVersion: 1`) is identical on 7.x
-  (7.32.0) and 8.0; standard fields (`subtype`/`visibility`/`data`/…) are top-level,
+- **`validatePortableGraph(data)`** — dry-run structural/version/endpoint check before import.
+- **Format** (`format:'brainy-portable-graph'`, `formatVersion: 1`) is identical on
+  7.x and 8.0; standard fields (`subtype`/`visibility`/`data`/…) are top-level,
   `metadata` is custom-only. Current-state (no generation history — that lives in
   `persist()`). Types exported from the package root.
+- **Naming:** the type is `PortableGraph` (with `PortableGraphEntity` /
+  `PortableGraphRelation`) — it is the portable interchange form of a graph, not a
+  backup (that role is `persist()`/`load()`). Renamed from the short-lived
+  `BackupData`/`'brainy-backup'` (introduced in 7.32.0, never adopted) with no
+  compatibility shim.
 - Guide: [docs/guides/export-and-import.md](docs/guides/export-and-import.md).
 
 ### Aggregation: distinctCount over any value type