feat(storage): add raw binary-blob primitive to every storage adapter

Introduce a first-class binary-blob storage primitive on the StorageAdapter
contract and implement it across all storage backends. This stores opaque byte
payloads verbatim instead of base64-in-JSON, eliminating the ~33% inflation and
full-materialization cost of the JSON envelope. It unblocks zero-copy,
mmap-able column-store segments and batch vector I/O at billion scale.

New methods (declared abstract on BaseStorageAdapter, the class that implements
StorageAdapter, and added to the StorageAdapter interface):

  saveBinaryBlob(key, data)    raw write, atomic on real filesystems
  loadBinaryBlob(key)          exact bytes, or null if absent
  deleteBinaryBlob(key)        idempotent (missing is ignored)
  getBinaryBlobPath(key)       real local fs path where one exists, else null

Shared key -> location convention across every adapter: the key's
"/"-separated segments nest under a `_blobs/` prefix and are suffixed with
`.bin`, e.g. "graph-lsm/source/sstable-123" ->
"<root>/_blobs/graph-lsm/source/sstable-123.bin". Blobs are not branch-scoped
(COW): they are immutable producer-managed segments.

Per-adapter behavior:
- FileSystemStorage: writes under <rootDir>/_blobs via tmp+rename; returns the
  real on-disk path so native code can mmap it directly. Path convention matches
  the existing MmapFileSystemStorage subclass byte-for-byte.
- S3CompatibleStorage / R2Storage / GcsStorage / AzureBlobStorage: put/get/delete
  raw octet-stream objects; getBinaryBlobPath returns null (remote stores have no
  local path).
- MemoryStorage: defensive-copied Map<string, Buffer>; null path; cleared on
  clear().
- OPFSStorage: stores raw bytes in the OPFS tree; null path.
- HistoricalStorageAdapter: read-only — save/delete throw; load resolves the
  blob from the historical commit tree; null path.

Tests: tests/unit/storage/binaryBlob.test.ts exercises save/load round-trip
(byte-identical, incl. non-UTF8 bytes), overwrite, delete-then-load, load-missing,
and getBinaryBlobPath behavior for all eight adapters. Cloud adapters run against
in-memory client fakes that drive the real adapter code; OPFS runs against an
in-memory FileSystem Access API mock; the historical adapter commits a blob into
a real COW tree. 59 new tests; full unit suite (1398 tests) green.

src/storage/adapters/r2Storage.ts

@@ -725,6 +725,110 @@ export class R2Storage extends BaseStorage {
     }
   }
 
+  // ===========================================================================
+  // Raw binary-blob primitive
+  // ===========================================================================
+
+  /**
+   * Map a blob key to its R2 object key under the shared `_blobs/` prefix, e.g.
+   * `"graph-lsm/source/sstable-123"` → `"_blobs/graph-lsm/source/sstable-123.bin"`.
+   *
+   * @param key - The blob key.
+   * @returns The R2 object key for the blob.
+   * @private
+   */
+  private blobObjectKey(key: string): string {
+    return `_blobs/${key}.bin`
+  }
+
+  /**
+   * Store a raw binary blob as an R2 object, writing the bytes verbatim with
+   * `application/octet-stream` content type. Overwrites any existing blob at the
+   * same key.
+   *
+   * @param key - The blob key.
+   * @param data - The exact bytes to store.
+   */
+  public async saveBinaryBlob(key: string, data: Buffer): Promise<void> {
+    await this.ensureInitialized()
+
+    const { PutObjectCommand } = await import('@aws-sdk/client-s3')
+    await this.s3Client!.send(
+      new PutObjectCommand({
+        Bucket: this.bucketName,
+        Key: this.blobObjectKey(key),
+        Body: data,
+        ContentType: 'application/octet-stream'
+      })
+    )
+  }
+
+  /**
+   * Load the raw bytes of the R2 blob object for `key`, or `null` if it does not
+   * exist.
+   *
+   * @param key - The blob key.
+   * @returns The blob bytes, or `null` if absent.
+   */
+  public async loadBinaryBlob(key: string): Promise<Buffer | null> {
+    await this.ensureInitialized()
+
+    try {
+      const { GetObjectCommand } = await import('@aws-sdk/client-s3')
+      const response = await this.s3Client!.send(
+        new GetObjectCommand({
+          Bucket: this.bucketName,
+          Key: this.blobObjectKey(key)
+        })
+      )
+      if (!response || !response.Body) {
+        return null
+      }
+      const bytes = await response.Body.transformToByteArray()
+      return Buffer.from(bytes)
+    } catch (error: any) {
+      if (error.name === 'NoSuchKey' || error.$metadata?.httpStatusCode === 404) {
+        return null
+      }
+      throw BrainyError.fromError(error, `loadBinaryBlob(${key})`)
+    }
+  }
+
+  /**
+   * Delete the R2 blob object for `key`. Missing objects are ignored.
+   *
+   * @param key - The blob key.
+   */
+  public async deleteBinaryBlob(key: string): Promise<void> {
+    await this.ensureInitialized()
+
+    try {
+      const { DeleteObjectCommand } = await import('@aws-sdk/client-s3')
+      await this.s3Client!.send(
+        new DeleteObjectCommand({
+          Bucket: this.bucketName,
+          Key: this.blobObjectKey(key)
+        })
+      )
+    } catch (error: any) {
+      if (error.name === 'NoSuchKey' || error.$metadata?.httpStatusCode === 404) {
+        return
+      }
+      throw new Error(`Failed to delete blob ${key}: ${error}`)
+    }
+  }
+
+  /**
+   * R2 is a remote object store with no local filesystem path to mmap, so this
+   * always returns `null`. Callers must use {@link loadBinaryBlob} instead.
+   *
+   * @param _key - The blob key (unused).
+   * @returns Always `null`.
+   */
+  public getBinaryBlobPath(_key: string): string | null {
+    return null
+  }
+
   /**
    * List all objects under a specific prefix in R2
    */