fix: preserve archive restores after pruning

test/test.ts

@@ -185,6 +185,19 @@ tap.test('should prune with keepLast=1', async () => {
   // Verify only 1 snapshot remains
   const snapshots = await repo.listSnapshots();
   expect(snapshots.length).toEqual(1);
+
+  // The remaining snapshot must still restore after partial-pack GC rewrites chunks.
+  const restoreStream = await repo.restore(snapshots[0].id, { item: 'config.tar' });
+  const chunks: Buffer[] = [];
+  await new Promise<void>((resolve, reject) => {
+    restoreStream.on('data', (chunk: Buffer) => chunks.push(chunk));
+    restoreStream.on('end', resolve);
+    restoreStream.on('error', reject);
+  });
+
+  const restored = Buffer.concat(chunks);
+  const expected = Buffer.alloc(32 * 1024, 'item-two-data');
+  expect(restored.equals(expected)).toBeTrue();
 });
 
 // ==================== Close ====================