feat: add multi-item ingest and Reed-Solomon parity

- Multi-item ingest: each item gets its own Unix socket, Rust processes
  them sequentially into a single snapshot with separate chunk lists
- Reed-Solomon parity: rs(20,1) erasure coding for pack file groups,
  enabling single-pack-loss recovery via parity reconstruction
- Repair now attempts parity-based recovery for missing pack files
- 16 integration tests + 12 Rust unit tests all pass

test/test.ts

@@ -117,6 +117,43 @@ tap.test('should restore data byte-for-byte', async () => {
   expect(restored.equals(expected)).toBeTrue();
 });
 
+// ==================== Multi-Item Ingest ====================
+
+tap.test('should ingest multiple items in one snapshot', async () => {
+  const data1 = Buffer.alloc(64 * 1024, 'item-one-data');
+  const data2 = Buffer.alloc(32 * 1024, 'item-two-data');
+
+  const snapshot = await repo.ingestMulti([
+    { stream: stream.Readable.from(data1), name: 'database.sql', type: 'database-dump' },
+    { stream: stream.Readable.from(data2), name: 'config.tar', type: 'volume-tar' },
+  ], { tags: { type: 'multi-test' } });
+
+  expect(snapshot).toBeTruthy();
+  expect(snapshot.items.length).toEqual(2);
+  expect(snapshot.items[0].name).toEqual('database.sql');
+  expect(snapshot.items[1].name).toEqual('config.tar');
+  expect(snapshot.items[0].size).toEqual(64 * 1024);
+  expect(snapshot.items[1].size).toEqual(32 * 1024);
+});
+
+tap.test('should restore specific item from multi-item snapshot', async () => {
+  const snapshots = await repo.listSnapshots({ tags: { type: 'multi-test' } });
+  expect(snapshots.length).toEqual(1);
+
+  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.length).toEqual(expected.length);
+  expect(restored.equals(expected)).toBeTrue();
+});
+
 // ==================== Verify ====================
 
 tap.test('should verify repository at quick level', async () => {
@@ -139,8 +176,9 @@ tap.test('should verify repository at full level', async () => {
 // ==================== Prune ====================
 
 tap.test('should prune with keepLast=1', async () => {
+  const snapshotsBefore = await repo.listSnapshots();
   const result = await repo.prune({ keepLast: 1 });
-  expect(result.removedSnapshots).toEqual(1);
+  expect(result.removedSnapshots).toEqual(snapshotsBefore.length - 1);
   expect(result.dryRun).toBeFalse();
 
   // Verify only 1 snapshot remains