fix: restore loadBinaryBlob fault-propagation (native column-store lockstep)

The Pass-1 hardening that makes loadBinaryBlob distinguish genuine absence
(ENOENT -> null) from a real fault (EIO/EACCES/... -> throw) was held out of
8.2.6 because the native accelerator's two column-store read sites still relied
on null-on-error. That accelerator release has now hardened those sites to
handle the throw (a faulted segment read marks the field unavailable and throws
a named error), so the fault-propagating form is restored and ships lockstep.

A present-but-unreadable index blob no longer masquerades as absent (which drove
needless rebuilds / empty reads). Adds the loadBinaryBlob leg to the blob
durability suite (absent -> null; present -> bytes; real fault -> throws).

tests/unit/storage/blob-save-durability.test.ts

@@ -74,3 +74,38 @@ describe('FileSystemStorage.saveBinaryBlob durable-write honesty (finding 13)',
     ).rejects.toMatchObject({ code: 'EIO' })
   })
 })
+
+describe('FileSystemStorage.loadBinaryBlob fault-propagation (finding 11; cor 3.0.13 lockstep)', () => {
+  let dir: string
+  let storage: any
+
+  beforeEach(async () => {
+    dir = fs.mkdtempSync(path.join(os.tmpdir(), 'brainy-blob-load-'))
+    storage = new FileSystemStorage(dir)
+    await storage.init()
+  })
+
+  afterEach(() => {
+    vi.restoreAllMocks()
+    fs.rmSync(dir, { recursive: true, force: true })
+  })
+
+  it('returns null for a genuinely absent blob (ENOENT)', async () => {
+    expect(await storage.loadBinaryBlob('never/written')).toBeNull()
+  })
+
+  it('reads back a present blob', async () => {
+    await storage.saveBinaryBlob('present/blob', Buffer.from([1, 2, 3]))
+    expect(await storage.loadBinaryBlob('present/blob')).toEqual(Buffer.from([1, 2, 3]))
+  })
+
+  it('propagates a real IO fault instead of masking it as absent', async () => {
+    await storage.saveBinaryBlob('present/blob', Buffer.from([1, 2, 3]))
+    vi.spyOn(fs.promises, 'readFile').mockRejectedValue(
+      Object.assign(new Error('disk fault'), { code: 'EIO' })
+    )
+    // A present-but-unreadable blob must NOT read as null (that drove needless
+    // native rebuilds / empty reads); cor 3.0.13's column-store handles the throw.
+    await expect(storage.loadBinaryBlob('present/blob')).rejects.toMatchObject({ code: 'EIO' })
+  })
+})