docs(8.0): export/import guide + api/README portable backup section

- New docs/guides/export-and-import.md (public) for the 8.0 surface:
  brain.export()/import(), Db composition (asOf/with time-travel + what-if export),
  selectors, options, BackupData v1 format, cross-version (7.x→8.0), VFS, and the
  generations/persist distinction. Documents only the implemented surface.
- api/README "Export & Import (portable) + Snapshots (native)": adds the portable
  brain.export()/import() round-trip alongside the native persist()/asOf() snapshot.

docs/api/README.md

@@ -1413,7 +1413,32 @@ await brain.import('https://api.example.com/data.json')
 
 ---
 
-### Export & Snapshots
+### Export & Import (portable) + Snapshots (native)
+
+**Portable graph backup** — `brain.export()` / `brain.import()` (`BackupData` v1, versioned
+JSON, partial-or-whole, cross-version). `export()` lives on the immutable `Db`, so it composes
+with `now()`/`asOf()`/`with()`:
+
+```typescript
+// Export part or all of the brain to a portable, versioned document
+const backup = await brain.export({ ids }, { includeVectors: true })
+
+// Restore it — import() routes a BackupData to the graph round-trip (merge by id)
+await otherBrain.import(backup, { onConflict: 'merge' })
+
+// Time-travel export (serialize a past generation) / what-if export (a speculative state)
+const past = await brain.asOf(gen)
+const asWas = await past.export({ collection: id })
+await past.release()
+await brain.now().with(ops).export({ ids })
+```
+
+Selectors: `{ ids }`, `{ collection }` (alias `memberOf`), `{ connected: { from, depth } }`,
+`{ vfsPath }`, predicate (`{ type, subtype, where, service }`), or whole brain (omit). See the
+**[Export & Import guide](../guides/export-and-import.md)**. Distinct from `brain.import(file)`
+(CSV/PDF/Excel/JSON ingestion — `import()` dispatches on whether you pass a `BackupData` or a file).
+
+**Native whole-brain snapshot** (generation-preserving, not portable JSON):
 
 ```typescript
 // Instant hard-link snapshot via the Db API